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

Protective measures against
#74502

Author: Thorsten Schütt

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp

@@ -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()) {
     auto Const = buildInstr(TargetOpcode::G_CONSTANT)
     .addDef(getMRI()->createGenericVirtualRegister(EltTy))
     .addCImm(&Val);
@@ -347,7 +350,10 @@ MachineInstrBuilder MachineIRBuilder::buildFConstant(const DstOp &Res,
 
   assert(!Ty.isPointer() && "invalid operand type");
 
-  if (Ty.isVector()) {
+  assert(!Ty.isScalableVector() &&
+         "unexpected scalable vector in buildFConstant");
+
+  if (Ty.isFixedVector()) {
     auto Const = buildInstr(TargetOpcode::G_FCONSTANT)
     .addDef(getMRI()->createGenericVirtualRegister(EltTy))
     .addFPImm(&Val);