[RISCV] Fix interleave crash on unary interleaves

We were crashing when lowering interleave shuffles like
(shuffle <0,3,1,4>, x:v4i8, y:v4i8)
Where it was technically an unary shuffle (both EvenSrc and OddSrc point
to the first operand), but the resulting extract_subvectors were out of
bounds.
This checks to make sure that the vectors being extracted are within
range.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D148647