[Local] Consider atomic loads from constant global as dead

Per the guidance in
https://llvm.org/docs/Atomics.html#atomics-and-ir-optimization,
an atomic load from a constant global can be dropped, as there can
be no stores to synchronize with. Any write to the constant global
would be UB.

IPSCCP will already drop such loads, but the main helper in Local
doesn't recognize this currently. This is motivated by D118387.

Differential Revision: https://reviews.llvm.org/D124241