Fix bugs in for loops and block exec

Author: alsonleej

src/cse-machine/__tests__/__snapshots__/cse-machine-runtime-context.ts.snap

@@ -532,7 +532,7 @@ exports[`Avoid unnescessary environment instruction 265`] = `5`;
 
 exports[`Avoid unnescessary environment instruction 266`] = `4`;
 
-exports[`Avoid unnescessary environment instruction 267`] = `4`;
+exports[`Avoid unnescessary environment instruction 267`] = `5`;
 
 exports[`Avoid unnescessary environment instruction 268`] = `4`;