[GlobalISel] Add a localizer pass for target to use

This reverts commit r299287 plus clean-ups.

The localizer pass is a helper pass that could be run at O0 in the GISel
pipeline to work around the deficiency of the fast register allocator.
It basically shortens the live-ranges of the constants so that the
allocator does not spill all over the place.

Long term fix would be to make the greedy allocator fast.

llvm-svn: 304051