[RISCV] Fix a bug accidentally introduced in e9311f9c

If we're lowering an e8 m8 shuffle and we have an index value greater than
255, we have no available space to generate an e16 index vector.  The
code had originally handled this correctly, but in a recent refactoring
I had moved the single source code above the check, and thus broke the
single source by accident.

I have a change on review to rework this (https://github.com/llvm/llvm-project/pull/79330), but for now, go with the most obvious fix.