[llvm] [AMDGPU] Simplify, fix and improve known bits for mbcnt (PR #104768)

[Matt Arsenault via llvm-commits]

================
@@ -15758,16 +15758,12 @@ void SITargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     case Intrinsic::amdgcn_mbcnt_hi: {
       const GCNSubtarget &ST =
           DAG.getMachineFunction().getSubtarget<GCNSubtarget>();
-      // These return at most the (wavefront size - 1) + src1
-      // As long as src1 is an immediate we can calc known bits
-      KnownBits Src1Known = DAG.computeKnownBits(Op.getOperand(2), Depth + 1);
-      unsigned Src1ValBits = Src1Known.countMaxActiveBits();
-      unsigned MaxActiveBits = std::max(Src1ValBits, ST.getWavefrontSizeLog2());
-      // Cater for potential carry
-      MaxActiveBits += Src1ValBits ? 1 : 0;
-      unsigned Size = Op.getValueType().getSizeInBits();
-      if (MaxActiveBits < Size)
-        Known.Zero.setHighBits(Size - MaxActiveBits);
+      // Wave64 mbcnt_lo returns at most 32 + src1. Otherwise these return at
+      // most 31 + src1.
----------------
arsenm wrote:

We fold mbcnt_hi out in wave32 in instcombine, so not sure how much use it is to handle here. But might as well for completeness.

Does this actually depend on the contents of exec_hi, or is it just ignored for wave32? 

https://github.com/llvm/llvm-project/pull/104768