[InstCombine] Convert some tests to opaque pointers (NFC)
These involved regenerating test checks. There are two significant
differences here:
1. With typed pointers we sometimes swapped gep and addrspacecast,
as a side-effect of other transforms. However, the current
direction is likely undesirable, and we should canonicalize in
the reverse direction instead (gep of ac, instead of ac of gep).
This should be done after typed pointers are removed, to avoid
conflicting transforms.
2. The "descaling" optimization isn't really compatible with
opaque pointers. This will be addressed longer-term by moving
away from type-based GEP, at which point the form with explicit
multiplications will be the canonical one.