i965/vec4: use the IR's execution size

i965/vec4: use the IR's execution size

In the vec4 backend the generator sets to 8 the execution size for all
instructions by default, however, to implement 64-bit floating-point we
will need to split certain instruction into smaller sizes so we need the
IR to convey this information like we do in the scalar backend. This patch
uses the execution size from the vec4 IR.

We will use this feature in a later patch when we implement a SIMD
splitting pass.

v2:
  - Drop the assertion on the execution size being 8 or 4 (Curro)
  - Use exec_size from backend_instruction (Curro)

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