RISC-V: optimize '(a >= 0) ? b : 0' to srai + andn, if compiling for Zbb
If-conversion is turning '(a >= 0) ? b : 0' into a branchless sequence
not a5,a0
srai a5,a5,63
and a0,a1,a5
missing the opportunity to combine the NOT and AND into an ANDN.
This adds a define_split to help the combiner reassociate the NOT with
the AND.