[SDAG] Relax conditions under stores of loaded values can be merged

Summary:

Allow consecutive stores whose values come from consecutive loads to
merged in the presense of other uses of the loads. Previously this was
disallowed as in general the merged load cannot be shared with the
other uses. Merging N stores into 1 may cause as many as N redundant
loads. However in the context of caching this should have neglible
affect on memory pressure and reduce instruction count making it
almost always a win.

Fixes PR32086.

Reviewers: spatel, jyknight, andreadb, hfinkel, efriedma

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30471

llvm-svn: 302712