nir: update opcode definitions for different bit sizes

nir: update opcode definitions for different bit sizes

Some opcodes need explicit bitsizes, and sometimes we need to use the
double version when constant folding.

v2: fix output type for u2f (Iago)

v3: do not change vecN opcodes to be float. The next commit will add
    infrastructure to enable 64-bit integer constant folding so this is isn't
    really necessary. Also, that created problems with source modifiers in
    some cases (Iago)

v4 (Jason):
  - do not change bcsel to work in terms of floats
  - leave ldexp generic

Squashed changes to handle different bit sizes when constant
folding since otherwise we would break the build.

v2:
- Use the bit-size information from the opcode information if defined (Iago)
- Use helpers to get type size and base type of nir_alu_type enum (Sam)
- Do not fallback to sized types to guess bit-size information. (Jason)

Squashed changes in i965 and gallium/nir drivers to support sized types.
These functions should only see sized types, but we can't make that change
until we make sure that nir uses the sized versions in all the relevant places.
A later commit will address this.

Signed-off-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>