i965/vec4: split d2x conversion and data gathering from one opcode to two explicit...

i965/vec4: split d2x conversion and data gathering from one opcode to two explicit ones

When doing a 64-bit to a smaller data type size conversion, the destination should
be aligned to 64-bits. Because of that, we need to gather the data after the
actual conversion.

Until now, these two operations were done by VEC4_OPCODE_FROM_DOUBLE but
now we split them explicitely in two different instructions:
VEC4_OPCODE_FROM_DOUBLE just do the conversion and
VEC4_OPCODE_PICK_LOW_32BIT will gather the data.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>