i965/fs: Lower the MULH virtual instruction.

i965/fs: Lower the MULH virtual instruction.

Translate MULH into the MUL/MACH sequence.  This does roughly the same
thing that nir_emit_alu() used to do but we can now handle 16-wide by
taking advantage of the SIMD lowering pass.  The force_sechalf
workaround near the bottom is required because the SIMD lowering pass
will emit instructions with non-zero quarter control and we need to
make sure we avoid that on integer arithmetic instructions with
implicit accumulator access due to a known hardware bug on IVB.

Reviewed-by: Matt Turner <mattst88@gmail.com>