Move STV(scalars_to_vector) RTL pass from i386 to target independent

[Dinar Temirbulatov via Gcc]

Hi,
I have observed that STV2 pass added ~20% on CPU2006 456.hmmer with mostly
by transforming V4SI operations. Looking at the pass itself, it looks like
it might be transformed into RTL architecture-independent, and the pass
deals only not wide integer operations. I think it might be useful on other
targets as well?
                                           Thanks, Dinar.