[DAGCombine] MergeConsecutiveStores - improve non-temporal load\store handling (PR42123)

This patch is the first step towards ensuring MergeConsecutiveStores correctly handles non-temporal loads\stores:

1 - When merging load\stores we must ensure that they all have the same non-temporal flag. This is unlikely to occur, but can in strange cases where we're storing at the end of one page and the beginning of another.

2 - The merged load\store node must retain the non-temporal flag.

Differential Revision: https://reviews.llvm.org/D62910

llvm-svn: 362723