tgsi: make SSE ADD instruction SOA-safe

tgsi: make SSE ADD instruction SOA-safe

To properly execute an instruction such as "ADD tmp, tmp.wzyx, foo;"
with SOA we (sometimes) need to put the results into temporaries before
writing the results to the destination register.

This patch fixes the ADD instruction but this needs to be done for
many more instructions.

Helps to fix piglit fp-long-alu test (fd.o bug 27989).