i965/fs: Stop using the LOAD_PAYLOAD instruction in lower_simd_width.

i965/fs: Stop using the LOAD_PAYLOAD instruction in lower_simd_width.

Instead of using the LOAD_PAYLOAD instruction (emitted through the
emit_transpose() helper that is no longer useful and this commit
removes) which had to be marked force_writemask_all in some cases,
emit a series of moves to apply proper channel enable signals to the
destination.  Until now lower_simd_width() had mainly been used to
lower things that invariably had a basic block-local temporary as
destination so it didn't seem like a big deal, but I found it to be
the reason for several Piglit regressions in my SIMD32 branch and
Igalia discovered the same issue independently while working on FP64
support.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>