[MIRParser] Allow register class names in the form of integer/scalar

Summary:
The current code cannot handle register class names like 'i32', which is
a valid register class name in WebAssembly. This patch removes special
handling for integer/scalar/pointer type parsing and treats them as
normal identifiers.

Reviewers: thegameg

Subscribers: jfb, dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D45948

llvm-svn: 331586