gallivm,ac: add function attributes at call sites instead of declarations

gallivm,ac: add function attributes at call sites instead of declarations

They can vary at call sites if the intrinsic is NOT a legacy SI intrinsic.
We need this to force readnone or inaccessiblememonly on some amdgcn
intrinsics.

This is only used with LLVM 4.0 and later. Intrinsics only used with
LLVM <= 3.9 don't need the LEGACY flag.

gallivm and ac code is in the same patch, because splitting would be
more complicated with all the LEGACY uses all over the place.

v2: don't change the prototype of lp_add_function_attr.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com> (v1)