[llvm] [PPC] Constrain register in VSPLT of XXSLDWI transform (PR #208005)

[Nikita Popov via llvm-commits]

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@@ -889,6 +889,8 @@ bool PPCMIPeephole::simplifyCode() {
             LLVM_DEBUG(dbgs() << "Changing splat immediate from " << SplatImm
                               << " to " << NewElem << " in instruction: ");
             LLVM_DEBUG(MI.dump());
+            if (!MRI->constrainRegClass(ShiftOp1, &PPC::VRRCRegClass))
+              llvm_unreachable("vrrc is subset of vsrc");
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nikic wrote:

I adjusted the message a bit to be clearer.

I'd also be fine with just bailing out. It seemed a bit cleaner to do it this way to me, because we don't expect this operation to fail.

https://github.com/llvm/llvm-project/pull/208005