[SROA] Handle PHI with multiple duplicate predecessors

Summary:
The verifier accepts PHI nodes with multiple entries for the
same basic block, as long as the value is the same.

As seen in PR37203, SROA did not handle such PHI nodes properly
when speculating loads over the PHI, since it inserted multiple
loads in the predecessor block and changed the PHI into having
multiple entries for the same basic block, but with different
values.

This patch teaches SROA to reuse the same speculated load for
each PHI duplicate entry in such situations.

Resolves: https://bugs.llvm.org/show_bug.cgi?id=37203

Reviewers: uabelho, chandlerc, hfinkel, bkramer, efriedma

Reviewed By: efriedma

Subscribers: dberlin, efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D46426

llvm-svn: 332577