ac: use llvm.amdgcn.fract intrinsic for nir_op_ffract

ac: use llvm.amdgcn.fract intrinsic for nir_op_ffract

Noticed with a Doom shader.

29077 shaders in 15096 tests
Totals:
SGPRS: 1282125 -> 1282133 (0.00 %)
VGPRS: 908716 -> 908616 (-0.01 %)
Spilled SGPRs: 24811 -> 24779 (-0.13 %)
Code Size: 49048176 -> 48936488 (-0.23 %) bytes
Max Waves: 244232 -> 244226 (-0.00 %)

Totals from affected shaders:
SGPRS: 229584 -> 229592 (0.00 %)
VGPRS: 163268 -> 163168 (-0.06 %)
Spilled SGPRs: 8682 -> 8650 (-0.37 %)
Code Size: 12819572 -> 12707884 (-0.87 %) bytes
Max Waves: 24398 -> 24392 (-0.02 %)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>