[llvm] [DAG] Improved ISD::SHL handling in isKnownToBeAPowerOfTwo (PR #181882)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> @arsenm Thanks. I'll start exploring.

known-pow2.ll already has at least one test that should improve - please regenerate it with the update_llc_test_checks.py script

https://github.com/llvm/llvm-project/pull/181882