i965/vec4: split DF instructions and later double its execsize in IVB/BYT

i965/vec4: split DF instructions and later double its execsize in IVB/BYT

We need to split DF instructions in two on IVB/BYT as it needs an
execsize 8 to process 4 DF values (one GRF in total).

v2:
- Rename helper and make it static inline function (Matt).
- Fix indention and add braces (Matt).

v3:
- Don't edit IR instruction when doubling exec_size (Curro)
- Add comment into the code (Curro).
- Manage ARF registers like the others (Curro)

v4:
- Add get_exec_type() function and use it to calculate the execution
  size.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Fix bogus 'type != BAD_FILE' check.  Take
  destination type as execution type where there is no valid source.
  Assert-fail if the deduced execution type is byte.  Clarify comment
  in get_lowered_simd_width().  Move SIMD width workaround outside of
  'if (...inst->size_written > REG_SIZE)' conditional block, since the
  problem should be independent of whether the amount of data written
  by the instruction is greater or lower than a GRF.  Drop redundant
  is_ivb_df definition.  Drop bogus inst->exec_size < 8 check.
  Simplify channel group assertion. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>