[DebugInfo][InstrRef] Don't fully propagate single assigned variables

If we only assign a variable value a single time, we can take a short-cut
when computing its location: the variable value is only valid up to the
dominance frontier of where the assignemnt happens. Past that point, there
are other predecessors from where the variable has no value, meaning the
variable has no location past that point.

This patch recognises this scenario, and avoids expensive SSA computation,
to improve compile-time performance.

Differential Revision: https://reviews.llvm.org/D117877