ac/nir: use llvm.amdgcn.rcp in ac_build_fdiv()

ac/nir: use llvm.amdgcn.rcp in ac_build_fdiv()

Instead of emitting 1.0 / x which includes a slow division that
LLVM doesn't always optimize even if the metadata is correctly set.

No pipeline-db changes with VEGA10/LLVM 9.

pipeline-db (VEGA10/LLVM 10):
Totals from affected shaders:
SGPRS: 6672 -> 6672 (0.00 %)
VGPRS: 6652 -> 6652 (0.00 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 561780 -> 561692 (-0.02 %) bytes
Max Waves: 1043 -> 1043 (0.00 %)

pipeline-db (VEGA10/LLVM 11 - 92744f62478):
Totals from affected shaders:
SGPRS: 84608 -> 83768 (-0.99 %)
VGPRS: 106768 -> 106636 (-0.12 %)
Spilled SGPRs: 1625 -> 1713 (5.42 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Code Size: 10850936 -> 10726712 (-1.14 %) bytes
Max Waves: 3152 -> 3180 (0.89 %)

LLVM 11 (master) is more affected than previous versions, but
based on the small impact with LLVM 9/10, I decided to emit it
unconditionally.

Cc: 20.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4326>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4326>