i965/vec4: Rename DF to/from F generator opcodes

i965/vec4: Rename DF to/from F generator opcodes

The opcodes are not specific for conversions to/from float since we need
the same for conversions to/from other 32-bit types. Rename the opcodes
accordingly and change the asserts to check the size of the types involved
instead.

v2:
- Rename to VEC4_OPCODE_TO_DOUBLE and VEC4_OPCODE_FROM_DOUBLE (Matt)

Reviewed-by: Matt Turner <mattst88@gmail.com>