Allow subtarget selection of the default MachineScheduler and document the interface.

The global registry is used to allow command line override of the
scheduler selection, but does not work well as the normal selection
API. For example, the same LLVM process should be able to target
multiple targets or subtargets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191071 91177308-0d34-0410-b5e6-96231b3b80d8
Lift alignment restrictions on load/store folding of VEXTRACTI128/VINSERTI128.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191073 91177308-0d34-0410-b5e6-96231b3b80d8