Add an RS_Split2 stage used for loop prevention.

This mechanism already exists, but the RS_Split2 stage makes it clearer.

When live range splitting creates ranges that may not be making
progress, they are marked RS_Split2 instead of RS_New. These ranges may
be split again, but only in a way that can be proven to make progress.

For local ranges, that means they must be split into ranges used by
strictly fewer instructions.

For global ranges, region splitting is bypassed and the RS_Split2
ranges go straight to per-block splitting.

llvm-svn: 135912