[ms] [llvm-ml] Add support for attempted register parsing

Summary:
Add a new method (tryParseRegister) that attempts to parse a register specification.

MASM allows the use of IFDEF <register>, as well as IFDEF <symbol>. To accommodate this, we make it possible to check whether a register specification can be parsed at the current location, without failing the entire parse if it can't.

Reviewers: thakis

Reviewed By: thakis

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73486