[3/11][POC][Clang][RISCV] Add typedef of the tuple type and define tuple type variant...

[3/11][POC][Clang][RISCV] Add typedef of the tuple type and define tuple type variant of vlseg2e32

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

Depends on D146873.

This is the 3rd patch of the patch-set. This patch originates from
D99593.

Note: 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 load is not removed, and only signed integer unit-strided
segment load of NF=2, EEW=32 is defined here.

When replacing the old intrinsics, the extra `IsTuple` parameter under
various places will be redundant and removed.

Authored-by: eop Chen <eop.chen@sifive.com>
Co-Authored-by: Hsiangkai Wang <kai.wang@sifive.com>
Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147731