ac/nir: Emit edge flag instructions conditionally.

ac/nir: Emit edge flag instructions conditionally.

They are not needed by RADV but will be needed by RadeonSI.

Fossil DB results on Sienna Cichlid (with NGGC on):
Totals from 56917 (44.24% of 128647) affected shaders:
VGPRs: 1982664 -> 1975936 (-0.34%); split: -0.43%, +0.09%
CodeSize: 152790880 -> 149510316 (-2.15%); split: -2.15%, +0.00%
MaxWaves: 1617984 -> 1621900 (+0.24%)
Instrs: 29272825 -> 28907038 (-1.25%); split: -1.26%, +0.01%
Latency: 128744182 -> 127565678 (-0.92%); split: -1.14%, +0.22%
InvThroughput: 20125915 -> 19805168 (-1.59%); split: -1.63%, +0.03%
VClause: 521312 -> 519804 (-0.29%); split: -0.77%, +0.48%
SClause: 688861 -> 688897 (+0.01%); split: -0.04%, +0.05%
Copies: 3205421 -> 3177799 (-0.86%); split: -1.68%, +0.82%
Branches: 1181457 -> 1183147 (+0.14%); split: -0.03%, +0.17%
PreVGPRs: 1626681 -> 1595406 (-1.92%)

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12998>