intel/compiler: validate region restrictions for half-float conversions

intel/compiler: validate region restrictions for half-float conversions

v2:
 - Consider implicit conversions in 2-src instructions too (Curro)
 - For restrictions that involve destination stride requirements
   only validate them for Align1, since Align16 always requires
   packed data.
 - Skip general rule for the dst/execution type size ratio for
   mixed float instructions on CHV and SKL+, these have their own
   set of rules that we'll be validated separately.

v3 (Curro):
 - Do not check src1 type in single-source instructions.
 - Check restriction on src1.
 - Remove invalid test.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>