[RISCV] Correct the EvenSrc/OddSrc computation in isInterleaveShuffle.

StartIndexes[0] Tells exactly which source element is in element 0,
the even source. Nothing needs to be swapped.

Since we're dealing with power of 2 vector lengths, StartIndexes[0]
is almost always even so the condition here was never true. The
exception is when we're interleaving two 1 element vectors. In that
case StartIndexes[0] could be 1.

We recently hit a failure from this on a pulldown. I don't have
the reduced reproducer yet and my naive attempts at making an
interleave of 1 element vectors produces a slideup instead so don't
go through this path.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D147268