radv: compact varyings after removing unused ones

radv: compact varyings after removing unused ones

It makes no sense to compact before, and the description of
nir_compact_varyings() confirms that.

Polaris10:
Totals from affected shaders:
SGPRS: 108528 -> 108128 (-0.37 %)
VGPRS: 74548 -> 74500 (-0.06 %)
Spilled SGPRs: 844 -> 814 (-3.55 %)
Code Size: 3007328 -> 2992932 (-0.48 %) bytes
Max Waves: 16019 -> 16009 (-0.06 %)

Vega10:
Totals from affected shaders:
SGPRS: 106088 -> 106232 (0.14 %)
VGPRS: 74652 -> 74700 (0.06 %)
Spilled SGPRs: 692 -> 658 (-4.91 %)
Code Size: 2967708 -> 2953028 (-0.49 %) bytes
Max Waves: 18178 -> 18162 (-0.09 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>