spirv,nir: lower frexp_exp/frexp_sig inside a new NIR pass

This lowering isn't needed for RADV because AMDGCN has two
instructions. It will be disabled for RADV in an upcoming series.

While we are at it, factorize a little bit.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
9 files changed