[AMDGPU][GISel] Fix MMO for raw/struct buffer access with non-constant offset

Codegen for the raw/struct buffer access intrinsics would update the
offset in the MMO to reflect the combined offset, if it was known to be
constant. If the combined offset was not known to be constant, or if
there was an index, it would set the offset in the MMO to 0. This is
unsafe because it makes it look like the access does not alias with
another access with a fixed non-zero offset.

Fix these cases by setting the pointer in the MMO to null, to reflect
the fact that we do not have any known IR value pointer + constant
offset for the access.

D106284 did this for SelectionDAG. This is the corresponding fix for
GlobalISel.

Differential Revision: https://reviews.llvm.org/D106451