Fix off-by-one error

The code in loop hoisting visiting a comma for a static init helper call
and static field access tried to clear the childrenHoistable bit for the
field, but had an off-by-one error and was instead clearing the bit for
the helper call, which could lead to missed legal helper call hoists.
Update that code to simply assert that the flag is already set for the
field (using the correct index), which must be true since the check is
guarded by a check that childIsCctorDependent is true for it.

Author: JosephTremoulet

src/jit/optimizer.cpp

@@ -6178,9 +6178,9 @@ bool Compiler::optHoistLoopExprsForTree(GenTreePtr        tree,
                         // with the static field reference.
                         treeIsCctorDependent = false;
                         // Hoisting the static field without hoisting the initialization would be
-                        // incorrect; unset childrenHoistable for the field to ensure this doesn't
-                        // happen.
-                        childrenHoistable[0] = false;
+                        // incorrect, make sure we consider the field (which we flagged as
+                        // cctor-dependent) non-hoistable.
+                        noway_assert(!childrenHoistable[childNum]);
                     }
                 }
             }