[SelectionDAGBuilder] Bugfix in visitInlineAsm()

In case of a virtual register tied to a phys-def, the register class needs to
be computed. Make sure that this works generally also with fast regalloc by
using TLI.getRegClassFor() whenever possible, and make only the case of
'Untyped' use getMinimalPhysRegClass().

Fixes https://bugs.llvm.org/show_bug.cgi?id=51699.

Review: Ulrich Weigand
Differential Revision: https://reviews.llvm.org/D109291