[llvm] [CodeGen] Add preliminary plumbing for `samesign` flag (PR #112354)

[via llvm-commits]

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@@ -1716,8 +1717,13 @@ bool TargetLowering::SimplifyDemandedBits(
       // if we don't care about FP signed-zero. The use of SETLT with FP means
       // that we don't care about NaNs.
       if (CC == ISD::SETLT && Op1.getValueType().isInteger() &&
-          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode())))
+          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode()))) {
+        if (Flags.hasSameSign()) {
+          Flags.setSameSign(false);
+          Op->setFlags(Flags);
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goldsteinn wrote:

This doesn't seem necessary for the plumbing and its not clear its correct to propagate the rest of the flags.

https://github.com/llvm/llvm-project/pull/112354