intel/fs: Introduce regioning lowering pass.

intel/fs: Introduce regioning lowering pass.

This legalization pass is meant to handle situations where the source
or destination regioning controls of an instruction are unsupported by
the hardware and need to be lowered away into separate instructions.
This should be more reliable and future-proof than the current
approach of handling CHV/BXT restrictions manually all over the
visitor.  The same mechanism is leveraged to lower unsupported type
conversions easily, which obsoletes the lower_conversions pass.

v2: Give conditional modifiers the same treatment as predicates for
    SEL instructions in lower_dst_modifiers() (Iago).  Special-case a
    couple of other instructions with inconsistent conditional mod
    semantics in lower_dst_modifiers() (Curro).

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>