[llvm] [X86] Use X86FixupInstTunings to select between (V)MOVSS/D and (V)BLENDPS/D (PR #143312)

[Simon Pilgrim via llvm-commits]

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@@ -221,8 +221,41 @@ bool X86FixupInstTuningPass::processInstruction(
   auto ProcessUNPCKPS = [&](unsigned NewOpc) -> bool {
     return ProcessUNPCKToIntDomain(NewOpc);
   };
+  
+  auto ProcessBLENDToMOV = [&](unsigned MovOpc) -> bool {
+    if (!MI.getOperand(NumOperands - 1).isImm() ||
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RKSimon wrote:

You shouldn't need this isImm() check - if we've gotten the operand number wrong then something is very broken, and getImm should assert.

https://github.com/llvm/llvm-project/pull/143312