aco: improve clamp optimization

aco: improve clamp optimization

Not sure why it checked the use count, it doesn't apply the constants.

pipeline-db (Navi):
Totals from affected shaders:
SGPRS: 269409 -> 269745 (0.12 %)
VGPRS: 238120 -> 238132 (0.01 %)
Spilled SGPRs: 305 -> 305 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 22908584 -> 22904672 (-0.02 %) bytes
Max Waves: 20217 -> 20217 (0.00 %)
Instructions: 4275312 -> 4263869 (-0.27 %)

pipeline-db (Vega):
Totals from affected shaders:
SGPRS: 155409 -> 155233 (-0.11 %)
VGPRS: 153072 -> 153072 (0.00 %)
Spilled SGPRs: 269 -> 269 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 14650824 -> 14650396 (-0.00 %) bytes
Max Waves: 9609 -> 9609 (0.00 %)
Instructions: 2762802 -> 2755517 (-0.26 %)

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2883>