DAG: Simplify demanded bits for truncating atomic_store (#90113)

It's really unfortunate that STORE and ATOMIC_STORE are separate
opcodes. This duplicates a basic simplify demanded for the truncating
case. This avoids some AMDGPU lit regressions in a future patch.

I'm not sure how to craft a test that exposes this without first
introducing the regressions by promoting half to i16.