Avoid fusing bitcasts with dynamic allocas if the amount-to-allocate

Avoid fusing bitcasts with dynamic allocas if the amount-to-allocate
might overflow.  Re-typing the alloca to a larger type (e.g. double)
hoists a shift into the alloca, potentially exposing overflow in the
expression.  rdar://problem/9265821

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132926 91177308-0d34-0410-b5e6-96231b3b80d8