[RISCV] Add side-effect-free vsetvli intrinsics

This patch introduces new intrinsics that enable the use of vsetvli in
contexts where only the returned vector length is of interest. The
pre-existing intrinsics are marked with side-effects, which prevents
even trivial optimizations on/across them.

These intrinsics are intended to be used in situations where the vector
length is fed in turn to RVV intrinsics or to vector-predication
intrinsics during loop vectorization, for example. Those codegen paths
ensure that instructions are generated with their own implicit vsetvli,
so the vector length and vtype can be relied upon to be correct.

No corresponding C builtins are planned at this stage, though that is a
possibility for the future if the need arises.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117910

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