mir-opt-level 4 is the new 3

Author: oli-obk

ci.sh

@@ -24,7 +24,7 @@ function run_tests {
   if [ -z "${MIRI_TEST_TARGET+exists}" ]; then
     # Only for host architecture: tests with optimizations (`-O` is what cargo passes, but crank MIR
     # optimizations up all the way).
-    MIRIFLAGS="-O -Zmir-opt-level=3" ./miri test --locked
+    MIRIFLAGS="-O -Zmir-opt-level=4" ./miri test --locked
   fi
 
   # On Windows, there is always "python", not "python3" or "python2".