[RISCV] Exploit register boundaries when lowering shuffle with exact vlen (#79072)

If we have a shuffle which is larger than m1, we may be able to split it
into a series of individual m1 shuffles. This patch starts with the
subcase where the mask allows a 1-to-1 mapping from source register to
destination register - each with a possible permutation of their own. We
can potentially extend this later, thought in practice this seems to
already catch a number of the most interesting cases.