i965/fs: Enforce common regioning restrictions by SIMD splitting.

i965/fs: Enforce common regioning restrictions by SIMD splitting.

This change addresses a number of hardware restrictions on the source
and destination regions and other execution controls of regular
FPU-like instructions that in some cases can be avoided by reducing
the execution size of the instruction.  Some of these restrictions
(e.g. the one about 3src instructions not supporting compression on
some hardware) are currently being worked around case by case in the
generator with ad-hoc splitting code that is buggy in several ways
(e.g. doesn't handle non-trivial execution controls which would break
SIMD32 code), but it seems cleaner to implement as many restrictions
as we can in a single lowering pass since that will allow us to
simplify some of the surrounding code considerably and also make sure
that we don't forget applying them in the future.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>