MIR: Simplify the implementation of `matchArg`

[qsctr]

When I first implemented matchArg, I heavily drew inspiration from SAWCentral.Crucible.LLVM.Overrides.matchArg, which also matches on expectedTy (but where expectedTy is an LLVM MemType instead). Looking back on the MIR version of matchArg with fresh eyes, however, you are right: there really isn't much of a point in including expectedTy, since we could just as well use the Ty stored inside of the TypeShape. It might be nice to simplify the implementation of this function by removing expectedTy.

(To be honest, I also wonder if the LLVM matchArg really needs to match on expectedTy either, but that is a task best left for later.)

Originally posted by @RyanGlScott in #2427 (comment)