[NFC] Divide tests into smaller files

This commit divides the large test files(over 30k lines) under clang/test/CodeGen/RISCV including:
rvv-intrinsics/vloxseg.c
rvv-intrinsics/vluxseg.c
rvv-intrinsics-overloaded/vloxseg.c
rvv-intrinsics-overloaded/vluxseg.c
into "non-masked" version and "masked" version which can reduce the test cases by 50% in a single file.

Differential Revision: https://reviews.llvm.org/D120967