tgsi: handle some src/dst aliasing in tgsi_sse2.c

tgsi: handle some src/dst aliasing in tgsi_sse2.c

Src/Dst aliasing (aka SOA dependencies) requires some care to ensure
intermediate results do not overwrite yet-to-be read source registers.

This change ensures that MOV/SWZ handle this correctly, which is poor but
no worse than the current tgsi_exec.c path.  Remove the fallback as there
is nothing to be gained correctness-wise between the two implementations now.

Fixing this properly looks like a bit of work in this code, but might be
easily achieved by sending destination writes to temporary storage.