nir: mind rounding mode on fadd, fsub, fmul and fma opcodes

nir: mind rounding mode on fadd, fsub, fmul and fma opcodes

According to Vulkan spec, the new execution modes affect only
correctly rounded SPIR-V instructions, which includes fadd, fsub and
fmul.

v2:
- Fix fmul, fsub and fadd round-to-zero definitions, they should use
  auxiliary functions to calculate the proper value because Mesa uses
  round-to-nearest-even rounding mode by default (Connor).

v3:
- Do an actual fused multiply-add at ffma (Connor).

v4:
- Simplify fadd and fmul for bit sizes < 64 (Connor).
- Do not use double ffma for 32 bits float (Connor).

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com> [v3]