i965/fs: Lower LOAD_PAYLOAD instructions of unsupported width.

i965/fs: Lower LOAD_PAYLOAD instructions of unsupported width.

Only per-channel LOAD_PAYLOAD instructions can be lowered, which
should cover everything that comes in from the front-end.

LOAD_PAYLOAD instructions used to construct actual message payloads
cannot be easily lowered because they contain headers and vectors of
variable type that aren't necessarily channel-aligned -- We shouldn't
find any of them in the program at SIMD lowering time though because
they're introduced during logical send lowering.

An alternative that may be worth considering would be to re-run the
SIMD lowering pass after LOAD_PAYLOAD lowering instead of this patch.

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