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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362723 91177308-0d34-0410-b5e6-96231b3b80d8