[GlobalISel][nfc] Remove unnecessary cast

[fdeazeve]

Summary

The getOperand method already returns a Constant when it is called on
a ConstantExpression, as such the cast is not needed. To prevent a type
mismatch between the different return statements of the lambda, the
lambda return type is explicitly provided.

[fdeazeve]

Changed commit message style

[fdeazeve]

@nikic Thank you for catching this in D130642!

[nikic]

LG

[fdeazeve]

The failing tests are timeouts that seem to happen in other reviews too

llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp

MachineInstrBuilder MachineIRBuilder::buildConstDbgValue(const Constant &C,
                                                          const MDNode *Expr) {
   assert(isa<DILocalVariable>(Variable) && "not a variable");
   assert(cast<DIExpression>(Expr)->isValid() && "not an expression");
   assert(
       cast<DILocalVariable>(Variable)->isValidLocationForIntrinsic(getDL()) &&
       "Expected inlined-at fields to agree");
   auto MIB = buildInstrNoInsert(TargetOpcode::DBG_VALUE);
 
-  auto *NumericConstant = [&] {
+  auto *NumericConstant = [&] () -> const Constant* {
     if (const auto *CE = dyn_cast<ConstantExpr>(&C))
       if (CE->getOpcode() == Instruction::IntToPtr)
-        return cast<const Constant>(CE->getOperand(0));
+        return CE->getOperand(0);
     return &C;
   }();
 
   if (auto *CI = dyn_cast<ConstantInt>(NumericConstant)) {
     if (CI->getBitWidth() > 64)
       MIB.addCImm(CI);
     else
       MIB.addImm(CI->getZExtValue());