[DebugInfo][InstrRef] Handle non-directly reachable DBG_PHIs in LiveDebugValues

Fixes: https://github.com/llvm/llvm-project/issues/62725

This patch fixes an error in which a DBG_INSTR_REF referring to a DBG_PHI in a
block that is not directly reachable from the entry block results in a crash
during LiveDebugValues. Note that this fix prevents a crash from occurring, but
will give undef locations to users of these PHIs even if a valid location exists.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D150707