[AMDGPU][GlobalISel] Fix unable to legalize G_FSHL error

[matejam]

Summary

Add implementation for lowering G_FSHL instruction in
LegalizerHelper::lower().

[arsenm]

Needs tests. Also I have this implemented already in https://reviews.llvm.org/D76500 (IIRC I think I didn't do the promotion/narrowing parts)

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

2925

This is not an immediate operand

2929

This isn't the equivalent operation

[foad]

D76500 has landed now, so maybe abandon this review?

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

     }
     return Legalized;
   }
+  case TargetOpcode::G_FSHL: {
+    Register DstReg = MI.getOperand(0).getReg();
+    Register SrcReg = MI.getOperand(1).getReg();
+    int64_t SrcImm = MI.getOperand(2).getImm();
+    LLT DstTy = MRI.getType(DstReg);
+
+    auto MIConst = MIRBuilder.buildConstant(DstTy, SrcImm);
+    MIRBuilder.buildShl(DstReg, SrcReg, MIConst->getOperand(0));
+    MI.eraseFromParent();
+    return Legalized;
+  }
   case TargetOpcode::G_FNEG: {
     Register Res = MI.getOperand(0).getReg();
     LLT Ty = MRI.getType(Res);