glsl: Fix warning related to tg4_offsets in release mode

Compiler can't know that array_size() of the offsets parameter in textureGatherOffsets is (at most) 4, so use a MIN2() to make the limit visible. Just adding an assert() gets ignored in Release builds.

This fixes the following warning in Release compilation:

  ../src/compiler/glsl/glsl_to_nir.cpp: In member function ‘virtual void {anonymous}::nir_visitor::visit(ir_texture*)’:
  ../src/compiler/glsl/glsl_to_nir.cpp:2453:41: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   2453 |                instr->tg4_offsets[i][j] = val;
        |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
  In file included from ../src/compiler/glsl/glsl_to_nir.h:31,
                   from ../src/compiler/glsl/glsl_to_nir.cpp:29:
  ../src/compiler/nir/nir.h:2470:11: note: at offset 8 into destination object ‘nir_tex_instr::tg4_offsets’ of size 8
   2470 |    int8_t tg4_offsets[4][2];
        |           ^~~~~~~~~~~
  ../src/compiler/glsl/glsl_to_nir.cpp:2453:41: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   2453 |                instr->tg4_offsets[i][j] = val;
        |                ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
  ../src/compiler/nir/nir.h:2470:11: note: at offset 9 into destination object ‘nir_tex_instr::tg4_offsets’ of size 8
   2470 |    int8_t tg4_offsets[4][2];
        |           ^~~~~~~~~~~

This is from: gcc (GCC) 14.1.1 20240522 (Red Hat 14.1.1-4).