nir/flrp: Lower flrp(a, b, c) differently if another flrp(a, _, c) exists

nir/flrp: Lower flrp(a, b, c) differently if another flrp(a, _, c) exists

This doesn't help on Intel GPUs now because we always take the
"always_precise" path first.  It may help on other GPUs, and it does
prevent a bunch of regressions in "intel/compiler: Don't always require
precise lowering of flrp".

No changes on any Intel platform.  Before a number of large rebases this
helped cycles in a couple shaders on Iron Lake and GM45.

Reviewed-by: Matt Turner <mattst88@gmail.com>