nir: fsub -> fadd_imm

nir: fsub -> fadd_imm

When doing nir_fsub(b, x, imm), we can negate the immediate value, and
replace the fsub with nir_fadd_imm() and get the same result. This makes
the code a bit shorter and easier to read.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23179>