[Thumb] Make load/store optimizer less conservative.

If it's safe to clobber the condition flags, we can do a few extra things:
it's then possible to reset the base register writeback using a SUBS, so
we can try to merge even if the base register isn't dead after the merged
instruction.

This is effectively a (heavily bug-fixed) rewrite of r208992.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218386 91177308-0d34-0410-b5e6-96231b3b80d8