i965/fs: Implement pass to lower instructions of unsupported SIMD width.

i965/fs: Implement pass to lower instructions of unsupported SIMD width.

This lowering pass implements an algorithm to expand SIMDN
instructions into a sequence of SIMDM instructions in cases where the
hardware doesn't support the original execution size natively for some
particular instruction.  The most important use-cases are:

 - Lowering send message instructions that don't support SIMD16
   natively into SIMD8 (several texturing, framebuffer write and typed
   surface operations).

 - Lowering messages that don't support SIMD8 natively into SIMD16
   (*cough*gen4*cough*).

 - 64-bit precision operations (e.g. FP64 and 64-bit integer
   multiplication).

 - SIMD32.

The algorithm works by splitting the sources of the original
instruction into chunks of width appropriate for the lowered
instructions, and then interleaving the results component-wise into
the destination of the original instruction.  The pass is controlled
by the get_lowered_simd_width() function that currently just returns
the original execution size making the whole pass a no-op for the
moment until some user is introduced.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
v2: Reverse order of the source transformations and split_inst emit
    call to make the code a bit easier to understand.