[RISCV] Teach computeKnownBits that vsetvli returns number less than 2^31.

This seems like a reasonable upper bound on VL. WG discussions for
the V spec would probably allow us to use 2^16 as an upper bound
on VLEN, but this is good enough for now.

This allows us to remove sext and zext if user happens to assign
the size_t result into an int and then uses it as a VL intrinsic
argument which is size_t.

Reviewed By: frasercrmck, rogfer01, arcbbb

Differential Revision: https://reviews.llvm.org/D101472