glsl: Handle mismatched array index precisions
It's not clear to me this is the 'right' fix, but this fixes the following assertion failure in glmark2-es2 -bterrain with all 16-bit CAPs enabled:
glmark2-es2-drm: ../src/compiler/nir/nir_builder.h:1190: nir_deref_instr* nir_build_deref_array(nir_builder*, nir_deref_instr*, nir_ssa_def*): Assertion index->bit_size == parent->dest.ssa.bit_size' failed.`