[llvm] [GlobalIsel][NFC] Harden MachineIRBuilder (PR #75465)

[Matt Arsenault via llvm-commits]

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@@ -314,7 +314,10 @@ MachineInstrBuilder MachineIRBuilder::buildConstant(const DstOp &Res,
   assert(EltTy.getScalarSizeInBits() == Val.getBitWidth() &&
          "creating constant with the wrong size");
 
-  if (Ty.isVector()) {
+  assert(!Ty.isScalableVector() &&
+         "unexpected scalable vector in buildConstant");
+
+  if (Ty.isFixedVector()) {
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arsenm wrote:

Changing this part seems redundant with the assert? 

https://github.com/llvm/llvm-project/pull/75465