nir/builder: Merge nir_[if]mov_alu into one nir_mov_alu helper

nir/builder: Merge nir_[if]mov_alu into one nir_mov_alu helper

Unless source modifiers are present, fmov and imov are the same.
There's no good reason for having two helpers.

Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Acked-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>