i965/fs: Handle 32-bit to 16-bit conversions

i965/fs: Handle 32-bit to 16-bit conversions

Conversions to 16-bit need having aligment between the 16-bit
and 32-bit types. So the conversion operations unpack 16-bit types
to with an stride=2 and then applies a MOV with the conversion.

v2 (Jason Ekstrand):
  - Avoid the general use of stride=2 for 16-bit register types.

v3 (Topi Pohjolainen)
  - Code style fix
   (Jason Ekstrand)
  - Now nir_op_f2f16 was renamed to nir_op_f2f16_undef
    because conversion to f16 with undefined rounding is explicit

Signed-off-by: Eduardo Lima <elima@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>