[llvm] [RISCV] computeKnownBits - add support for RISCVISD::VECREDUCE_ADD_VL (PR #210493)

[Sabona Abdi via llvm-commits]

sabonaoabdi wrote:

> Looks like this needs a test. Are you able to add one?

added some tests that cover both branches of the VL bounds and varying start values. Issue is that nothing calls `computeKnownBits` on VECREDUCE_ADD_VL in a way that reaches final codegen since `RISCVISD::VMV_X_S` has no known bits support to recurse through.

https://github.com/llvm/llvm-project/pull/210493