[4/11][POC][Clang][RISCV] Define tuple type variant of vsseg2e32

For the cover letter of this patch-set, please checkout D146872.

Depends on D147731.

This is the 4th patch of the patch-set.

This patch is a proof-of-concept and will be extended to full coverage
in the future. Currently, the old non-tuple unit-stride segment store is
not removed, and only signed integer unit-strided segment store of NF=2,
EEW=32 is defined here.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147774