i965/vec4: split VEC4_OPCODE_FROM_DOUBLE into one opcode per destination's type

i965/vec4: split VEC4_OPCODE_FROM_DOUBLE into one opcode per destination's type

This way we can set the destination type as double to all these new opcodes,
avoiding any optimizer's confusion that was happening before.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Drop no_spill workaround originally needed due to
  the bogus destination type of VEC4_OPCODE_FROM_DOUBLE. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>