glsl: dont lower precision for textureGatherOffsets

glsl: dont lower precision for textureGatherOffsets

textureGatherOffsets always takes a highp array of constants. As
per the discussion in [1] trying to lower the precision results in segfault
later on in the compiler as textureGatherOffsets will end up being passed
a temp when its expecting a constant as required by the spec.

[1] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16547#note_1393704

Fixes: b83f4b9fa23d ("glsl: Add an IR lowering pass to convert mediump operations to 16-bit")

Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18101>