[SveEmitter] Add builtins to insert/extract subvectors from tuples (svget/svset)
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
Differential Revision: https://reviews.llvm.org/D81464