[RegAllocGreedy] Take last chance recoloring into account in split and assign

Summary:
This is a follow-up to r353988 where tryEvict was extended to take last
chance recoloring into account. Now we do the same thing for trySplit and
tryAssign.

Now we always pass a "FixedRegisters" argument to canEvictInterference and
tryEvict so it doesn't need to have a default value anymore.

The need for this was found long ago in an out-of-tree target.
Unfortunately I don't have a reproducer for an in-tree target.

Reviewers: qcolombet, rudkx

Reviewed By: qcolombet, rudkx

Subscribers: rudkx, MatzeB, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58376

llvm-svn: 354439