i965/fs: add helper to retrieve instruction execution type

i965/fs: add helper to retrieve instruction execution type

The execution data size is the biggest type size of any instruction
operand.

We will use it to know if the instruction deals with DF, because in Ivy
we need to double the execution size and regioning parameters.

v2:
- Fix typo in commit log (Matt)
- Use static inline function instead of fs_inst's method (Curro).
- Define the result as a constant (Curro).
- Fix indentation (Matt).
- Add braces to nested control flow (Matt).

v3 (Curro):
- Add get_exec_type() and other auxiliary functions and use them to
  calculate its size.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
[ Francisco Jerez: Fix bogus 'type != BAD_FILE' check.  Fix deduced
  execution type for integer vector types.  Take destination type as
  execution type where there is no valid source.  Assert-fail if the
  deduced execution type is byte.  Move into brw_ir_fs.h header for
  consistency with the VEC4 back-end. ]
Reviewed-by: Francisco Jerez <currojerez@riseup.net>