i965/vec4: Fix UBO loads for 64-bit data

i965/vec4: Fix UBO loads for 64-bit data

We need to emit 2 32-bit load messages to load a full dvec4. If only
1 or 2 double components are needed dead-code-elimination will remove
the second one.

We also need to shuffle the result of the 32-bit messages to form
valid 64-bit SIMD4x2 data.

v2:
 - use byte_offset() instead of offset() (Iago)
 - keep the const. offset as an immediate like the original code did (Juan)

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