[SveEmitter] Add builtins to insert/extract subvectors from tuples (svget/svset)
For example:
svint32_t svget4(svint32x4_t tuple, uint64_t imm_index)
returns the subvector at `index`, which must be in range `0..3`.
svint32x3_t svset3(svint32x3_t tuple, uint64_t index, svint32_t vec)
returns a tuple vector with `vec` inserted into `tuple` at `index`,
which must be in range `0..2`.
Reviewers: c-rhodes, efriedma
Reviewed By: c-rhodes
Tags: #clang
Differential Revision: https://reviews.llvm.org/D81464