[Machine Combiner] Refactor machine reassociation code to be target-independent.

No functional change intended.
Patch by Haicheng Wu <haicheng@codeaurora.org>!

http://reviews.llvm.org/D12887
PR24522

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248164 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed