make optimized-test-run a bit more like what cargo does

ci.sh

@@ -24,8 +24,9 @@ function run_tests {
 
   ./miri test --locked
   if [ -z "${MIRI_TEST_TARGET+exists}" ]; then
-    # Only for host architecture: tests with MIR optimizations
-    MIRIFLAGS="-Z mir-opt-level=3" ./miri test --locked
+    # 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
   fi
 
   # On Windows, there is always "python", not "python3" or "python2".

tests/compile-fail/data_race/dealloc_read_race_stack.rs

@@ -1,6 +1,5 @@
 // ignore-windows: Concurrency on Windows is not supported yet.
-// compile-flags: -Zmiri-disable-isolation -Zmir-opt-level=0
-// With optimizations (in particular #78360), the span becomes much worse, so we disable them.
+// compile-flags: -Zmiri-disable-isolation
 
 use std::thread::{spawn, sleep};
 use std::ptr::null_mut;

tests/compile-fail/data_race/dealloc_write_race_stack.rs

@@ -1,6 +1,5 @@
 // ignore-windows: Concurrency on Windows is not supported yet.
-// compile-flags: -Zmiri-disable-isolation -Zmir-opt-level=0
-// With optimizations (in particular #78360), the span becomes much worse, so we disable them.
+// compile-flags: -Zmiri-disable-isolation
 
 use std::thread::{spawn, sleep};
 use std::ptr::null_mut;