st/mesa: tex offsets can't be in a const or 2d-indexed

st/mesa: tex offsets can't be in a const or 2d-indexed

All consts are now implicitly 2d (they set .Dimension), so trigger
asserts. Also, the texture offset can't handle any sort of 2d indexing.
While this could be tacked on, this seems unnecessary, just move it off
into a separate temp.

Fixes assertion failure in
tests/spec/arb_gpu_shader5/compiler/builtin-functions/fs-gatherOffset-uniform-offset.frag

Note that this was an issue even before the const-always-2d thing, since
there was no detection of when even a proper second dimension was used,
e.g. for UBO or geom/tess inputs.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>