[DebugInstrRef] Memoize variable order during sorting (NFC)

Instead of constructing DebugVariables and looking up the order
in the comparison function, compute the order upfront and then sort
a vector of (order, instr).

This improves compile-time by -0.4% geomean on CTMark ReleaseLTO-g.

Differential Revision: https://reviews.llvm.org/D117575