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

llvm-svn: 320000