[ValueTracking] Compute known FPClass from signbit idiom (#80740)

This patch improves `computeKnownFPClass` by using context-sensitive
information from `DomConditionCache`.
The motivation of this patch is to optimize the following case found in
[fmt/format.h](https://github.com/fmtlib/fmt/blob/e17bc67547a66cdd378ca6a90c56b865d30d6168/include/fmt/format.h#L3555-L3566):
```
define float @test(float %x, i1 %cond) {
  %i32 = bitcast float %x to i32
  %cmp = icmp slt i32 %i32, 0
  br i1 %cmp, label %if.then1, label %if.else

if.then1:
  %fneg = fneg float %x
  br label %if.end

if.else:
  br i1 %cond, label %if.then2, label %if.end

if.then2:
  br label %if.end

if.end:
  %value = phi float [ %fneg, %if.then1 ], [ %x, %if.then2 ], [ %x, %if.else ]
  %ret = call float @llvm.fabs.f32(float %value)
  ret float %ret
}
```
We can prove the sign bit of %value is always zero. Then the fabs can be
eliminated.

This pattern also exists in cpython/duckdb/oiio/openexr.

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=f82e0809ba12170e2f648f8a1ac01e78ef06c958&to=041218bf5491996edd828cc15b3aec5a59ddc636&stat=instructions:u


|stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang|
|--|--|--|--|--|--|--|
|-0.00%|+0.01%|+0.00%|-0.03%|+0.00%|+0.00%|+0.02%|

1 files changed, 10 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 6c42fac..cc1d5b7 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4272,7 +4272,7 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
   Value *LHS;
   uint64_t ClassVal = 0;
   const APFloat *CRHS;
-  // TODO: handle sign-bit check idiom
+  const APInt *RHS;
   if (match(Cond, m_FCmp(Pred, m_Value(LHS), m_APFloat(CRHS)))) {
     auto [CmpVal, MaskIfTrue, MaskIfFalse] = fcmpImpliesClass(
         Pred, *CxtI->getParent()->getParent(), LHS, *CRHS, LHS != V);
@@ -4282,6 +4282,15 @@ static void computeKnownFPClassFromCond(const Value *V, Value *Cond,
                              m_Value(LHS), m_ConstantInt(ClassVal)))) {
     FPClassTest Mask = static_cast<FPClassTest>(ClassVal);
     KnownFromContext.knownNot(CondIsTrue ? ~Mask : Mask);
+  } else if (match(Cond, m_ICmp(Pred, m_ElementWiseBitCast(m_Value(LHS)),
+                                m_APInt(RHS)))) {
+    bool TrueIfSigned;
+    if (!isSignBitCheck(Pred, *RHS, TrueIfSigned))
+      return;
+    if (TrueIfSigned == CondIsTrue)
+      KnownFromContext.signBitMustBeOne();
+    else
+      KnownFromContext.signBitMustBeZero();
   }
 }