MachineCombiner Pass for selecting faster instruction sequence on AArch64

Re-commit of r214832,r21469 with a work-around that
avoids the previous problem with gcc build compilers

The work-around is to use SmallVector instead of ArrayRef
of basic blocks in preservesResourceLen()/MachineCombiner.cpp



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215151 91177308-0d34-0410-b5e6-96231b3b80d8