i965/fs: Simplify texture destination fixups

i965/fs: Simplify texture destination fixups

There are a few different fixups that we have to do for texture
destinations that re-arrange channels, fix hardware vs. API mismatches, or
just shrink the result to fit in the NIR destination.  These were all being
done in a somewhat haphazard manner.  This commit replaces all of the
shuffling with a single LOAD_PAYLOAD operation at the end and makes it much
easier to insert fixups between the texture instruction itself and the
LOAD_PAYLOAD.

Shader-db results on Haswell:

   total instructions in shared programs: 6227035 -> 6226669 (-0.01%)
   instructions in affected programs: 19119 -> 18753 (-1.91%)
   helped: 85
   HURT: 0

   total cycles in shared programs: 56491626 -> 56476126 (-0.03%)
   cycles in affected programs: 672420 -> 656920 (-2.31%)
   helped: 92
   HURT: 42