When considering folding a bitcast of an alloca into the alloca itself,
make sure we don't shrink the amount of memory being allocated, or
things rapidly go sideways.
rdar://13324424
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176547 91177308-0d34-0410-b5e6-96231b3b80d8
