Fix a problem I introduced in r187029 where we would over-eagerly
schedule an alloca for another iteration in SROA. This only showed up
with a mixture of promotable and unpromotable selects and phis. Added
a test case for this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187031 91177308-0d34-0410-b5e6-96231b3b80d8