Handle allocations that, even after removing dead uses, still have more than

Handle allocations that, even after removing dead uses, still have more than
one use (but one is a cast).  This handles the very common case of:

 X = alloc [n x byte]
 Y = cast X to somethingbetter
 seteq X, null

In order to avoid infinite looping when there are multiple casts, we only
allow this if the xform is strictly increasing the alignment of the
allocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23961 91177308-0d34-0410-b5e6-96231b3b80d8