Revert the business end of r164634, and replace it with a different fix. The

Revert the business end of r164634, and replace it with a different fix. The
reason we were getting two of the same alloca is because of a memmove/memcpy
which had the same alloca in both the src and dest. Now we detect that case
directly. This has the same testcase as before, but fixes a clang test
CodeGenObjC/exceptions.m which runs clang -O2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164636 91177308-0d34-0410-b5e6-96231b3b80d8