Fix race condition for C++ < C++11
Fix race condition for C++ < C++11

Was crashing simd_op_check occasionally.

I'm a little suprised that Halide isn't thread-unsafe elsewhere
(e.g. unique_name, shared pointer reference counts) but I guess
if you never pass an Expr, Func, etc across threads, it all works.


Former-commit-id: 6fd134f84cf2393960c152071eefdb5a9334e6cd
darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide