[LegalizeTypes][VP] Add splitting support for binary VP ops

This patch extends D107904's introduction of vector-predicated (VP)
operation legalization to include vector splitting.

When the result of a binary VP operation needs splitting, all of its
operands are split in kind. The two operands and the mask are split as
usual, and the vector-length parameter EVL is "split" such that the low
and high halves each execute the correct number of elements.

Tests have been added to the RISC-V target to show splitting several
scenarios for fixed- and scalable-vector types. Without support for
`umax` (e.g. in the `B` extension) the generated code starts to branch.
Ideally a cost model would prevent their insertion in the first place.

Through these tests many opportunities for better codegen can be seen:
combining known-undef VP operations and for constant-folding operations
on `ISD::VSCALE`, to name but a few.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D107957