[llvm] [GlobalISel] Fix crash in isConstantOrConstantSplatVectorFP for non-r… (PR #208657)

[Matt Arsenault via llvm-commits]

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@@ -1542,6 +1542,8 @@ llvm::isConstantOrConstantSplatVector(MachineInstr &MI,
 std::optional<APFloat>
 llvm::isConstantOrConstantSplatVectorFP(MachineInstr &MI,
                                         const MachineRegisterInfo &MRI) {
+  if (!MI.getOperand(0).isReg())
+    return std::nullopt;
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arsenm wrote:

> Wouldn't the semantics of this be slightly odd given that some indices are defs and some are uses?

The intent would only be to use it with defs 

https://github.com/llvm/llvm-project/pull/208657