i965/fs: Lower arithmetic instructions with register regions of unsupported width.

This extends the SIMD lowering pass to enforce the hardware limitation
that no directly-addressed source may read more than 2 physical GRFs.
One can easily go over this limit when doing 64-bit arithmetic
(e.g. FP64 or extended-precision integer MULs) or SIMD32, so it's nice
to be able to just emit an instruction of the intended execution size
from the visitor and let the lowering pass deal with this restriction
transparently.

Some hardware arithmetic instructions are not handled here, including
all instructions that use the accumulator implicitly (which the SIMD
lowering pass deliberately doesn't handle), instructions with
non-per-channel sources (e.g. LINE or PLANE) and SEND-like
instructions, which need special handling most likely as virtual
opcodes.

Reviewed-by: Connor Abbott <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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