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

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.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D96423