MachineCombiner Pass for selecting faster instruction
 sequence on AArch64

Re-commit of r214669 without changes to test cases
LLVM::CodeGen/AArch64/arm64-neon-mul-div.ll and
LLVM:: CodeGen/AArch64/dp-3source.ll
This resolves the reported compfails of the original commit.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214832 91177308-0d34-0410-b5e6-96231b3b80d8