Use shader_arb_add_instruction_modifiers instead. This avoids calling the
fixup function from each single instruction handler to handle shifts. It does
not yet get rid of the modifier handler in each instruction because we don't
want a separate line if we can just append _SAT to the instruction name.

It shows that our current behavior (enabling coord_replace on all
units) is correct.

This is needed to raise the number of advertised constants to the GL
limit.  The ARB assembler ususally does not optimize away unused
constants, so we have to do this.