Following on from D139466 which added support for dead vreg defs, this patch adds support for "undef" defs of subregs. Use this to regenerate checks for amx-greedy-ra-spill-shape.ll which previously required manual tweaks to the autogenerated checks to fix an EXPENSIVE_CHECKS failure; see commit 8b7c1fbd9647a5a6ef246a6b5b2543ea0f5a2337