[DebugInfo] Only perform TypeSize -> unsigned cast when necessary

[joechrisellis]

Summary

This commit moves a line in SelectionDAGBuilder::handleDebugValue to
avoid implicitly casting a TypeSize object to an unsigned earlier than
necessary. It was possible that we bail out of the loop before the value
is ever used, which means we could create a superfluous TypeSize
warning.

[DavidTruby]

LGTM!

[david-arm]

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

1324

I'm not sure this is the right thing to do here. I realise you want a quick workaround, but I think the better workaround for now is to simply move this down until after the bail out below and add a TODO that this needs updating for scalable vectors perhaps?

[joechrisellis]

Address @david-arm's comments.

[joechrisellis]

ACK, fair point!

[david-arm]

LGTM!

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

if (VMI != FuncInfo.ValueMap.end()) {
       if (RFV.occupiesMultipleRegs()) {
         unsigned Offset = 0;
         unsigned BitsToDescribe = 0;
         if (auto VarSize = Var->getSizeInBits())
           BitsToDescribe = *VarSize;
         if (auto Fragment = Expr->getFragmentInfo())
           BitsToDescribe = Fragment->SizeInBits;
         for (auto RegAndSize : RFV.getRegsAndSizes()) {
-          unsigned RegisterSize = RegAndSize.second;
           // Bail out if all bits are described already.
           if (Offset >= BitsToDescribe)
             break;
+          // TODO: handle scalable vectors.
+          unsigned RegisterSize = RegAndSize.second;
           unsigned FragmentSize = (Offset + RegisterSize > BitsToDescribe)
               ? BitsToDescribe - Offset
               : RegisterSize;
           auto FragmentExpr = DIExpression::createFragmentExpression(
               Expr, Offset, FragmentSize);
           if (!FragmentExpr)
               continue;
           SDV = DAG.getVRegDbgValue(Var, *FragmentExpr, RegAndSize.first,