aco: don't use constant_bits when not applying constants

This is incorrect for 16bit integer operands.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35020>

src/amd/compiler/aco_optimizer_postRA.cpp

@@ -638,7 +638,7 @@ try_combine_dpp(pr_opt_ctx& ctx, aco_ptr<Instruction>& instr)
          continue;
 
       bool input_mods = can_use_input_modifiers(ctx.program->gfx_level, instr->opcode, i) &&
-                        get_operand_type(instr, i).constant_bits() == 32;
+                        get_operand_type(instr, i).bit_size == 32;
       bool mov_uses_mods = mov->valu().neg[0] || mov->valu().abs[0];
       if (((dpp8 && ctx.program->gfx_level < GFX11) || !input_mods) && mov_uses_mods)
          continue;