[RISCV] Lower the shufflevector equivalent of vector.splice

We can lower a vector splice to a vslidedown and a vslideup.

The majority of the matching code here came from X86's code for matching
PALIGNR and VPALIGND/Q.

The slidedown and slideup lowering don't really require it to be concatenation,
but it happened to be an interesting pattern with existing analysis code I
could use.

This helps with cases where the scalar loop optimizer forwarded a load
result from a previous loop iteration. For example, this happens if the
loop uses x[i] and x[i+1] on the same iteration. The scalar optimizer
will forward x[i+1] load from the previous loop to satisfy x[i] on this
loop. When this get vectorized it results in one element of a vector
being forwarded from the previous loop to be concatenated with elements
loaded on this iteration.

Whether that's more efficient than doing a shifted loaded or reloading
the single scalar and using vslide1up is an interesting question.
But that's not something the backend can help with.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D119039

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