[CodeGen] std::optional::value => operator*/operator->

value() has undesired exception checking semantics and calls
__throw_bad_optional_access in libc++. Moreover, the API is unavailable without
_LIBCPP_NO_EXCEPTIONS on older Mach-O platforms (see
_LIBCPP_AVAILABILITY_BAD_OPTIONAL_ACCESS).

This fixes LLVMMIRParser, LLVMGlobalISel, LLVMAsmPrinter, LLVMSelectionDAG.