[RISCV] Add isel patterns to select vsub_vx intrinsic to vadd.vi if it uses a...

[RISCV] Add isel patterns to select vsub_vx intrinsic to vadd.vi if it uses a small enough immediate

Also modify the simm5_plus1 check because Imm-1 is UB if Imm happens
to be INT64_MIN. I don't think the compiler would optimize based on that in this
usage, but it could fail UBSan or -ftrapv.

Reviewed By: HsiangKai, frasercrmck

Differential Revision: https://reviews.llvm.org/D99637