middle-end: don't create LC-SSA PHI variables for PHI nodes who dominate loop

As the testcase shows, when a PHI node dominates the loop there is no new
definition inside the loop.  As such there would be no PHI nodes to update.

When we maintain LCSSA form we create an intermediate node in between the two
loops to thread alongt the value.  However later on when we update the second
loop we don't have any PHI nodes to update and so adjust_phi_and_debug_stmts
does nothing.   This leaves us with an incorrect phi node.  Normally this does
nothing and just gets ignored.  But in the case of the vUSE chain we end up
corrupting the chain.

As such whenever a PHI node's argument dominates the loop, we should remove
the newly created PHI node after edge redirection.

The one exception to this is when the loop has been versioned.  In such cases
the versioned loop may not use the value but the second loop can.

When this happens and we add the loop guard unless the join block has the PHI
it can't find the original value for use inside the guard block.

The next refactoring in the series moves the formation of the guard block
inside peeling itself.  Here we have all the information and wouldn't
need to re-create it later.

gcc/ChangeLog:

	PR tree-optimization/111860
	* tree-vect-loop-manip.cc (slpeel_tree_duplicate_loop_to_edge_cfg):
	Remove PHI nodes that dominate loop.

gcc/testsuite/ChangeLog:

	PR tree-optimization/111860
	* gcc.dg/vect/pr111860.c: New test.

1 files changed, 16 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/vect/pr111860.c b/gcc/testsuite/gcc.dg/vect/pr111860.c
new file mode 100644
index 0000000..36f0774
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr111860.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+
+int optimize_path_n, optimize_path_d;
+int *optimize_path_d_0;
+extern void path_threeOpt( long);
+void optimize_path() {
+  int i;
+  long length;
+  i = 0;
+  for (; i <= optimize_path_n; i++)
+    optimize_path_d = 0;
+  i = 0;
+  for (; i < optimize_path_n; i++)
+    length += optimize_path_d_0[i];
+  path_threeOpt(length);
+}