Simplify vector ((VCE (a cmp b ? -1 : 0)) < 0) ? c : d to just (VCE ((a cmp b) ? (VCE c) : (VCE d))).

When I'm working on PR112443, I notice there's some misoptimizations:
after we fold _mm{,256}_blendv_epi8/pd/ps into gimple, the backend
fails to combine it back to v{,p}blendv{v,ps,pd} since the pattern is
too complicated, so I think maybe we should hanlde it in the gimple
level.

The dump is like

  _1 = c_3(D) >= { 0, 0, 0, 0 };
  _2 = VEC_COND_EXPR <_1, { -1, -1, -1, -1 }, { 0, 0, 0, 0 }>;
  _7 = VIEW_CONVERT_EXPR<vector(32) char>(_2);
  _8 = VIEW_CONVERT_EXPR<vector(32) char>(b_6(D));
  _9 = VIEW_CONVERT_EXPR<vector(32) char>(a_5(D));
  _10 = _7 < { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  _11 = VEC_COND_EXPR <_10, _8, _9>;

It can be optimized to

  _1 = c_2(D) >= { 0, 0, 0, 0 };
  _6 = VEC_COND_EXPR <_1, b_5(D), a_4(D)>;

since _7 is either -1 or 0, the selection of _7 < 0 ? _8 : _9 should
be euqal to _1 ? b : a as long as TYPE_PRECISION of the component type
of the second VEC_COND_EXPR is less equal to the first one.
The patch add a gimple pattern to handle that.

gcc/ChangeLog:

	* match.pd (VCE (a cmp b ? -1 : 0) < 0) ? c : d ---> (VCE ((a
	cmp b) ? (VCE:c) : (VCE:d))): New gimple simplication.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512vl-blendv-3.c: New test.
	* gcc.target/i386/blendv-3.c: New test.

3 files changed, 74 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 4d554ba..5deddd5 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -5190,6 +5190,28 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
  (if (optimize_vectors_before_lowering_p () && types_match (@0, @3))
   (vec_cond (bit_and @0 (bit_not @3)) @2 @1)))
 
+/*  ((VCE (a cmp b ? -1 : 0)) < 0) ? c : d is just
+    (VCE ((a cmp b) ? (VCE c) : (VCE d))) when TYPE_PRECISION of the
+    component type of the outer vec_cond is greater equal the inner one.  */
+(for cmp (simple_comparison)
+ (simplify
+  (vec_cond
+    (lt (view_convert@5 (vec_cond@6 (cmp@4 @0 @1)
+				    integer_all_onesp
+				    integer_zerop))
+	  integer_zerop) @2 @3)
+  (if (VECTOR_INTEGER_TYPE_P (TREE_TYPE (@0))
+       && VECTOR_INTEGER_TYPE_P (TREE_TYPE (@5))
+       && !TYPE_UNSIGNED (TREE_TYPE (@5))
+       && VECTOR_TYPE_P (TREE_TYPE (@6))
+       && VECTOR_TYPE_P (type)
+       && tree_int_cst_le (TYPE_SIZE (TREE_TYPE (type)),
+			   TYPE_SIZE (TREE_TYPE (TREE_TYPE (@6))))
+       && TYPE_SIZE (type) == TYPE_SIZE (TREE_TYPE (@6)))
+   (with { tree vtype = TREE_TYPE (@6);}
+     (view_convert:type
+       (vec_cond @4 (view_convert:vtype @2) (view_convert:vtype @3)))))))
+
 /* c1 ? c2 ? a : b : b  -->  (c1 & c2) ? a : b  */
 (simplify
  (vec_cond @0 (vec_cond:s @1 @2 @3) @3)
diff --git a/gcc/testsuite/gcc.target/i386/avx512vl-blendv-3.c b/gcc/testsuite/gcc.target/i386/avx512vl-blendv-3.c
new file mode 100644
index 0000000..2777e72
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/avx512vl-blendv-3.c
@@ -0,0 +1,6 @@
+/* { dg-do compile } */
+/* { dg-options "-mavx512vl -mavx512bw -O2" } */
+/* { dg-final { scan-assembler-times {vp?blendv(?:b|p[sd])[ \t]*} 6 } } */
+/* { dg-final { scan-assembler-not {vpcmp} } } */
+
+#include "blendv-3.c"
diff --git a/gcc/testsuite/gcc.target/i386/blendv-3.c b/gcc/testsuite/gcc.target/i386/blendv-3.c
new file mode 100644
index 0000000..fa0fb06
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/blendv-3.c
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-options "-mavx2 -O2" } */
+/* { dg-final { scan-assembler-times {vp?blendv(?:b|p[sd])[ \t]*} 6 } } */
+/* { dg-final { scan-assembler-not {vpcmp} } } */
+
+#include <immintrin.h>
+
+__m256i
+foo (__m256i a, __m256i b, __m256i c)
+{
+  return _mm256_blendv_epi8 (a, b, ~c < 0);
+}
+
+__m256d
+foo1 (__m256d a, __m256d b, __m256i c)
+{
+  __m256i d = ~c < 0;
+  return _mm256_blendv_pd (a, b, (__m256d)d);
+}
+
+__m256
+foo2 (__m256 a, __m256 b, __m256i c)
+{
+  __m256i d = ~c < 0;
+  return _mm256_blendv_ps (a, b, (__m256)d);
+}
+
+__m128i
+foo4 (__m128i a, __m128i b, __m128i c)
+{
+  return _mm_blendv_epi8 (a, b, ~c < 0);
+}
+
+__m128d
+foo5 (__m128d a, __m128d b, __m128i c)
+{
+  __m128i d = ~c < 0;
+  return _mm_blendv_pd (a, b, (__m128d)d);
+}
+
+__m128
+foo6 (__m128 a, __m128 b, __m128i c)
+{
+  __m128i d = ~c < 0;
+  return _mm_blendv_ps (a, b, (__m128)d);
+}