https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68620
--- Comment #1 from Christophe Lyon <clyon at gcc dot gnu.org> --- The problem happens when trying to extract subregs from V4HF. On big-endian, CANNOT_CHANGE_MODE_CLASS returns 1, which at some point makes xpart = operand_subword (x, i, 1, mode); (in emit_move_multi_word) return NULL in xpart, later trigging the assert.
