CodeGen: Fix invalid bitcasts for memcpy

CreateCoercedLoad/CreateCoercedStore assumes pointer argument of
memcpy is in addr space 0, which is not correct and causes invalid
bitcasts for triple amdgcn---amdgiz.

It is fixed by using alloca addr space instead.

Differential Revision: https://reviews.llvm.org/D40806


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@320000 91177308-0d34-0410-b5e6-96231b3b80d8