glsl: Skip rewriting instructions in opt_cpe when unnecessary.

glsl: Skip rewriting instructions in opt_cpe when unnecessary.

Previously, opt_copy_propagation_elements would always rewrite the
instruction stream, even if was the same thing as before.  In order to
report progress correctly, we'll need to bail if the suggested
replacement is identical (or equivalent) to the original code.

This also introduced unnecessary noop swizzles, as far as I can tell.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>