[LIR] General refactoring to simplify code and the ease future code review

This is a resubmission of r256336 which was reverted in r256361. The issue was the lack of the invariant check of the memset value in processLooMemSet().

The original message:

Move several checks into isLegalStores. Also, delineate between those stores that are memset-able and those that are memcpy-able.

llvm-svn: 256783