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

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

http://reviews.llvm.org/D15683
Patch by Haicheng Wu <haicheng@codeaurora.org>!

llvm-svn: 256336