[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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304051
91177308-0d34-0410-b5e6-
96231b3b80d8
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