[SROA] Correctly invalidate analyses when dead instructions deleted

Summary:
SROA can fail in rewriting alloca but still rewrite a phi resulting
in dead instruction elimination. The Changed flag was not being set
correctly, resulting in downstream passes using stale analyses.
The included test case will assert during the second BDCE pass as a
result.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39921

llvm-svn: 318677