TTI: Improve default costs for addrspacecast

TTI: Improve default costs for addrspacecast

For some reason multiple places need to do this, and the variant the
loop unroller and inliner use was not handling it.

Also, introduce a new wrapper to be slightly more precise, since on
AMDGPU some addrspacecasts are free, but not no-ops.

llvm-svn: 362436