[llvm] fix typo in comment

Author: gchatelet

llvm/include/llvm/CodeGen/MachineFrameInfo.h

@@ -177,7 +177,7 @@ class MachineFrameInfo {
     /// If true, the object has been zero-extended.
     bool isZExt = false;
 
-    /// If true, the object has been zero-extended.
+    /// If true, the object has been sign-extended.
     bool isSExt = false;
 
     uint8_t SSPLayout;