nir/opt_algebraic: Add fmul(a, 2.0) -> fadd(a, a)

Investigated for Bifrost, but should be the same or cheaper for any
reasonable architecture. For the compilers I maintain:

* Bifrost - fadd can be scheduled 2x as frequently as ffma, there is no
  fmul separate from ffma.
* Midgard - fadd x, x is used as a canonical form, again for easier
  scheduling.
* AGX - fmul and fadd are both native ops, but fmul is heavier weight
  (unknown whether this is a performance issue or just power
   consumption). Also saves a move / uniform file slot for the constant.

Since floating point multiplication is inherently more expensive than
addition, presumably this is a win for everyone else too.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>