[X86][DAG] Switch X86 Target to post-legalized store merge

Move store merge to happen after intrinsic lowering to allow lowered
stores to be merged.

Some regressions due in MergeConsecutiveStores to missing
insert_subvector that are addressed in follow up patch.

Reviewers: craig.topper, efriedma, RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34559

llvm-svn: 310710