This resubmits r284436 and r284437, which were reverted in
r284462 as they were breaking the AArch64 buildbot.
The breakage on AArch64 turned out to be a miscompile which is
still not fixed, but is actively tracked at llvm.org/pr30748.
This resubmission re-writes the code in a way so as to make the
miscompile not happen.
llvm-svn: 285483