[VP][SelectionDAG] Add a target-configurable EVL operand type

[VP][SelectionDAG] Add a target-configurable EVL operand type

This patch adds a way for the target to configure the type it uses for
the explicit vector length operands of VP SDNodes. The type must be a
legal integer type (there is still no target-independent legalization of
this operand) and must currently be at least as big as i32, the type
used by the IR intrinsics. An implicit zero-extension takes place on
targets which choose a larger type. All VP nodes should be created with
this type used for the EVL operand.

This allows 64-bit RISC-V to avoid custom legalization of all VP nodes,
keeping them in their target-independent form for that bit longer.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D103027