Relax fast register allocator related test cases; NFC

- Relex hard coded registers and stack frame sizes
- Some test cleanups
- Change phi-dbg.ll to match on mir output after phi elimination instead
  of going through the whole codegen pipeline.

This is in preparation for https://reviews.llvm.org/D52010
I'm committing all the test changes upfront that work before and after
independently.

llvm-svn: 345532