assertion in MachineCopyPropagation::isNopCopy


I'm hitting an assertion in MachineCopyPropagation::isNopCopy:

if (Src == PreviousSrc) {
   assert(Def == PreviousDef);
   return true;

This code compares two COPY instruction to see whether they are effectively "the same". The assert assumes that, if the sources are the same register then the destinations must be as well.
My instructions look like this, however:

   %V2<def> = COPY %V6
   %V6<def> = COPY %V2_LO

(isNopCopy is called with src and dest of the first instruction switched.)
So dst2 == src1 but src2 is a subregister of dst1.
Is there some assumption that size(src) == size(dst)?
This second COPY, with size(src) != size(dst) was generated in InstrEmitter::EmitSubregNode at

// Create the extract_subreg machine instruction.

So it's not out-of-tree code.
Am I missing something or should the assert condition be part of the "if" condition instead?