tgsi: fix SSE code emit for XPD

tgsi: fix SSE code emit for XPD

Rearrange things so that the writes to the dest registers happen
after we've fetched/used all src registers.

The problematic instruction was:  XPD TEMP[2].xyz, TEMP[0], TEMP[2];
Note that the dst reg is also a src reg.

This fixes bad shading with progs/glsl/bump.c since Eric's changes to the
Mesa program optimizer in commit d6690ce15fb8c7c6abf1bc0d847c1d2da2c33904.
The optimizer rearranges some registers so we occasionally wind up with
something like the above.