ARMLoadStoreOpt: Merge subs/adds into LDRD/STRD; Factor out common code

Re-apply of r241928 which had to be reverted because of the r241926
revert.

This commit factors out common code from MergeBaseUpdateLoadStore() and
MergeBaseUpdateLSMultiple() and introduces a new function
MergeBaseUpdateLSDouble() which merges adds/subs preceding/following a
strd/ldrd instruction into an strd/ldrd instruction with writeback where
possible.

Differential Revision: http://reviews.llvm.org/D10676

llvm-svn: 242743