[SimplifyCFG] Pass context instruction to isSafeToSpeculativelyExecute() (#109132)
Pass speculation target and assumption cache to
isSafeToSpeculativelyExecute() calls.
This allows speculating based on dereferenceable/align assumptions, but
the primary motivation here is to avoid regressions from planned changes
to fix https://github.com/llvm/llvm-project/issues/108854.