[llvm] [X86] Use GFNI for vXi8 shifts/rotates (PR #89115)

[Simon Pilgrim via llvm-commits]

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@@ -31345,7 +31392,8 @@ static SDValue LowerBITREVERSE(SDValue Op, const X86Subtarget &Subtarget,
   // If we have GFNI, we can use GF2P8AFFINEQB to reverse the bits.
   if (Subtarget.hasGFNI()) {
     MVT MatrixVT = MVT::getVectorVT(MVT::i64, NumElts / 8);
-    SDValue Matrix = DAG.getConstant(0x8040201008040201ULL, DL, MatrixVT);
+    SDValue Matrix =
+        DAG.getConstant(getGFNICtrlImm(ISD::BITREVERSE), DL, MatrixVT);
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RKSimon wrote:

This shouldn't be necessary - if we've managed to call LowerBITREVERSE with `Op.getOpcode() != ISD::BITREVERSE` then we have bigger problems.

https://github.com/llvm/llvm-project/pull/89115