[loop-deletion] Overly defensive with undef-ing dbg.values.

Explicitly inserting undef is overly defensive. Any values computed
nside the loop that are referenced by dbg.values should naturally
become undef when the loop is deleted, and all other values that
are loop invariant must be preserved.

Reviewed By: scott.linder

Differential Revision: https://reviews.llvm.org/D153539