[SIL] Add source formal type to checked_cast_br.

It is necessary for opaque values where for casts that will newly start
out as checked_cast_brs and be lowered to checked_cast_addr_brs, since
the latter has the source formal type, IRGen relies on being able to
access it, and there's no way in general to obtain the source formal
type from the source lowered type.

lib/SILOptimizer/Transforms/SILCodeMotion.cpp

@@ -956,7 +956,7 @@ enum OperandRelation {
 ///
 /// bb1:
 ///  %3 = unchecked_enum_data %0 : $Optional<X>, #Optional.Some!enumelt
-///  checked_cast_br [exact] %3 : $X to $X, bb4, bb5 // id: %4
+///  checked_cast_br [exact] X in %3 : $X to $X, bb4, bb5 // id: %4
 ///
 /// bb4(%10 : $X):                                    // Preds: bb1
 ///  strong_release %10 : $X