LiveIntervals: add removeRegUnit

Summary:
See D22198 for the motivation: We have a pass that uses LiveIntervals anyway,
and there is now a requirement to track a physical register that is not
usually tracked at this point of the compilation. The pass also introduces
instructions that affect this physical register, but we want to preserve
LiveIntervals.

Rather than add brittle and rarely exercised code to keep the tracking of
the physical register intact, we want to just remove the corresponding
LiveRange -- it didn't exist before anyway, and subsequent passes don't
expect it to be there.

Reviewers: MatzeB, arsenm

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D22801

llvm-svn: 278920