[InstCombine] exclude bitcast of ppc_fp128 in icmp signbit fold

Based on the post-commit comments for rG0f56bbc, there might
be a problem with this transform:

(bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0

...and the ppc_fp128 data type, so conservatively bypass if we
are bitcasting a ppc_fp128.

We might be able to account for endian or other differences to
enable this for PowerPC again if that is useful.

Differential Revision: https://reviews.llvm.org/D77642

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2763,11 +2763,13 @@ static Instruction *foldICmpBitCast(ICmpInst &Cmp,
         return new ICmpInst(Pred, X, ConstantInt::getNullValue(X->getType()));
 
     // If this is a sign-bit test of a bitcast of a casted FP value, eliminate
-    // the FP cast because the FP cast does not change the sign-bit.
+    // the FP extend/truncate because that cast does not change the sign-bit.
+    // This is true for all standard IEEE-754 types and the X86 80-bit type.
+    // The sign-bit is always the most significant bit in those types.
     const APInt *C;
     bool TrueIfSigned;
-    if (match(Op1, m_APInt(C)) && Bitcast->hasOneUse() &&
-        isSignBitCheck(Pred, *C, TrueIfSigned)) {
+    if (!BCSrcOp->getType()->isPPC_FP128Ty() && match(Op1, m_APInt(C)) &&
+        Bitcast->hasOneUse() && isSignBitCheck(Pred, *C, TrueIfSigned)) {
       if (match(BCSrcOp, m_FPExt(m_Value(X))) ||
           match(BCSrcOp, m_FPTrunc(m_Value(X)))) {
         // (bitcast (fpext/fptrunc X)) to iX) < 0 --> (bitcast X to iY) < 0

llvm/test/Transforms/InstCombine/icmp.ll

@@ -3700,8 +3700,9 @@ define i1 @signbit_bitcast_fpext_extra_use(float %x, i64* %p) {
 
 define i1 @signbit_bitcast_fpext_ppc_fp128(float %x) {
 ; CHECK-LABEL: @signbit_bitcast_fpext_ppc_fp128(
-; CHECK-NEXT:    [[TMP1:%.*]] = bitcast float [[X:%.*]] to i32
-; CHECK-NEXT:    [[S4:%.*]] = icmp slt i32 [[TMP1]], 0
+; CHECK-NEXT:    [[S2:%.*]] = fpext float [[X:%.*]] to ppc_fp128
+; CHECK-NEXT:    [[S3:%.*]] = bitcast ppc_fp128 [[S2]] to i128
+; CHECK-NEXT:    [[S4:%.*]] = icmp slt i128 [[S3]], 0
 ; CHECK-NEXT:    ret i1 [[S4]]
 ;
   %s2 = fpext float %x to ppc_fp128