[Debugify] Strip llvm.mir.debugify metadata

We don't strip llvm.mir.debugify metadata in `llvm::stripDebugifyMetadata`. This
may lead to incorrect number of lines and variables in the metadata when we run
debugify twice, e.g. -run-pass=mir-debugify,...,mir-strip-debug,...,mir-debugify.

Differential Revision: https://reviews.llvm.org/D138417