update_mir_test_checks.py: match undef vreg subreg definitions (llvm#66627)

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
8b7c1fb

Author: jayfoad