[TargetLowering][RISCV] Allow truncation when checking if the arguments of a setcc are splats.

We're just trying to canonicalize here and won't be using the constant
value returned.

The attached test changes are because we were previously commuting
a seteq X, (splat_vector 0) because we also have (sub 0, X). The
0 is larger than the element type so we don't detect it as a splat
without the AllowTruncation flag. By preventing the commute we are
able to match it to the vmseq.vx instruction during isel. We only
look for constants on the RHS in isel.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D123256