[Debug-info][InstrRef] Avoid an unnecessary map ordering

We keep a record of substitutions between debug value numbers post-isel,
however we never actually look them up until the end of compilation. As a
result, there's nothing gained by the collection being a std::map. This
patch downgrades it to being a vector, that's then sorted at the end of
compilation in LiveDebugValues.

Differential Revision: https://reviews.llvm.org/D105029