[DAGCombine] Improve Load-Store Forwarding

Summary:
Extend analysis forwarding loads from preceeding stores to work with
extended loads and truncated stores to the same address so long as the
load is fully subsumed by the store.

Hexagon's swp-epilog-phis.ll and swp-memrefs-epilog1.ll test are
deleted as they've no longer seem to be relevant.

Reviewers: RKSimon, rnk, kparzysz, javed.absar

Subscribers: sdardis, nemanjai, hiraditya, atanasyan, llvm-commits

Differential Revision: https://reviews.llvm.org/D49200

llvm-svn: 344142