[libre-riscv-dev] SV branch operation

[Luke Kenneth Casson Leighton]

ok that's working with a preliminary unit test that sets up 2
predicated registers, does a vector-compare, and if all are true it
will branch.  results are recorded in *rs2*'s predication (if one
exists).

the unit tests for this one need a lot more entries, i modified the
existing test to check by hand that various combinations of input
worked, needs to be a bit more formal than that, in principle it's
good.

i think the next main thing will be compressed branch, then, really,
that's the majority of the work done: it'll just be about the
corner-cases, check that zeroing works, check that vectors
specifically of length 1 work: really it's about the unit tests,
there.

the next *really* major task would be to add in support for
non-default element widths.  separate message about that.

l.