i965/vec4: split double-precision SEL

i965/vec4: split double-precision SEL

There is a hardware bug affecting compressed double-precision SEL
instructions in align16 mode by which they won't read predication mask
properly. The bug does not affect other predicated instructions
and it does not affect SEL in Align1 mode either. This was found
empirically and verified by Curro in the simulator.

Fix this by splitting double-precision SEL in Align16 mode to use an
execution size of 4.

v2: Check that the dst type is 64-bit, since we can have 16-wide single
    precision bcsel instructions that also write 2 registers.

v3: Replace bcsel by SEL in all the comments as bcsel is the nir opcode
but SEL is the actual assembly instruction (Matt).

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