x86-64: adjust REX-prefix part of SSE2AVX test

Before altering how build_modrm_byte() works, arrange for this part of
the testcase to actually use distinguishable source and destination
register numbers, such that incorrect propagation of, in particular, the
high bit encodings (from REX to VEX) can be noticed (in turn
specifically assertions [not] triggering in the respective code).