glsl_to_tgsi: set correct register type for array and structure elements

glsl_to_tgsi: set correct register type for array and structure elements

This fixes an issue where glsl_to_tgsi_visior::get_opcode() would emit the
wrong opcode because the register type was GLSL_TYPE_ARRAY/STRUCT instead of
GLSL_TYPE_FLOAT/INT/UINT/BOOL, so the function would use the float opcodes for
operations on integer or boolean values dereferenced from an array or
structure.  Assertions have been added to get_opcode() to prevent this bug
from reappearing in the future.