Implement operands for the 'if' statement.

This CL also includes two other minor changes:
- change the implemented syntax from 'if (cond)' to 'if cond', as specified by MLIR spec.
- a minor fix to the implementation of the ForStmt.

PiperOrigin-RevId: 210618122
diff --git a/lib/IR/Verifier.cpp b/lib/IR/Verifier.cpp
index deda932..48f3643 100644
--- a/lib/IR/Verifier.cpp
+++ b/lib/IR/Verifier.cpp
@@ -372,7 +372,7 @@
 
     // TODO: check that operation is not a return statement unless it's
     // the last one in the function.
-    // TODO: check that loop bounds are properly formed.
+    // TODO: check that loop bounds and if conditions are properly formed.
     if (verifyReturn())
       return true;