nir/lower_load_const_to_scalar: support doubles and multiple bit sizes
nir/lower_load_const_to_scalar: support doubles and multiple bit sizes

v2 (Sam):
- Add assert to detect bitsizes differents than 32 and 64 (Jason).

Reviewed-by: Jason Ekstrand <68c46a606457643eab92053c1c05574abb26f861@jlekstrand.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler