i965/vec4: Add a shuffle_64bit_data helper

i965/vec4: Add a shuffle_64bit_data helper

SIMD4x2 64bit data is stored in register space like this:

r0.0:DF  x0 y0 z0 w0
r1.0:DF  x1 y1 z1 w1

When we need to write data such as this to memory using 32-bit write
messages we need to shuffle it in this fashion:

r0.0:DF  x0 y0 x1 y1
r0.1:DF  z0 w0 z1 w1

and emit two 32-bit write messages, one for r0.0 at base_offset
and another one for r0.1 at base_offset+16.

We also need to do the inverse operation when we read using 32-bit messages
to produce valid SIMD4x2 64bit data from the data read. We can achieve this
by aplying the exact same shuffling to the data read, although we need to
apply different channel enables since the layout of the data is reversed.

This helper implements the data shuffling logic and we will use it in
various places where we read and write 64bit data from/to memory.

v2 (Curro):
  - Use the writemask helper and don't assert on the original writemask
    being XYZW.
  - Use the Vec4 IR builder to simplify the implementation.

v3 (Iago):
  - Use byte_offset() instead of offset().

v3:
  - Fix typo (Matt)
  - Clarify the example and fix indention (Matt).

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