[NFC][Clang][RISCV] Reduce boilerplate when determining prototype for segment loads

[NFC][Clang][RISCV] Reduce boilerplate when determining prototype for segment loads

No functionality change of the RVV builtin and compiler intrinsics is intended
in this patch.

This patch gathers scattered comments for the segment load builtin/intrinsics
and its variants (e.g. segment unit-stride load, segment strided load) into a
single paragraph under riscv_vector.td.

This patch also tries to reduce one level of the if-statements as the push_back
are essentially the same actions but differs in index based on the the value
of the policy attributes and whether the intrinsic is masked.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D140662