[GlobalISel] Allow handling G_MUL with libcalls in LegalizerHelper

[lewis-revill]

Summary

If we need to handle G_MUL using a libcall (EG in RISC-V when multiplication is not supported), this should be possible using LegalizerHelper. This patch simply adds the missing cases to make sure this is possible.

[craig.topper]

This looks to have been done in D130987

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

do {                                                                         \
     case 128:                                                                  \
       return RTLIB::LibcallPrefix##128;                                        \
     default:                                                                   \
       llvm_unreachable("unexpected size");                                     \
     }                                                                          \
   } while (0)
 
   switch (Opcode) {
+  case TargetOpcode::G_MUL:
+    RTLIBCASE_INT(MUL_I);
   case TargetOpcode::G_SDIV:
     RTLIBCASE_INT(SDIV_I);
   case TargetOpcode::G_UDIV:
     RTLIBCASE_INT(UDIV_I);
   case TargetOpcode::G_SREM:
     RTLIBCASE_INT(SREM_I);
   case TargetOpcode::G_UREM:
     RTLIBCASE_INT(UREM_I);

 LegalizerHelper::libcall(MachineInstr &MI, LostDebugLocObserver &LocObserver) {
   LLT LLTy = MRI.getType(MI.getOperand(0).getReg());
   unsigned Size = LLTy.getSizeInBits();
   auto &Ctx = MIRBuilder.getMF().getFunction().getContext();
 
   switch (MI.getOpcode()) {
   default:
     return UnableToLegalize;
+  case TargetOpcode::G_MUL:
   case TargetOpcode::G_SDIV:
   case TargetOpcode::G_UDIV:
   case TargetOpcode::G_SREM:
   case TargetOpcode::G_UREM:
   case TargetOpcode::G_CTLZ_ZERO_UNDEF: {
     Type *HLTy = IntegerType::get(Ctx, Size);
     auto Status = simpleLibcall(MI, MIRBuilder, Size, HLTy);
     if (Status != Legalized)