[LICM] Disable thread-safety checks in single-thread model

If the single-thread model is used, or the
-licm-force-thread-model-single flag is specified, skip checks
related to thread-safety. This means that store promotion for
conditionally executed stores only requires proof of
dereferenceability and writability, but not of thread-safety. For
example, this enables promotion of stores to (non-constant) globals,
as well as captured allocas.

Fixes https://github.com/llvm/llvm-project/issues/50537.

Differential Revision: https://reviews.llvm.org/D130466