[DebugInfo][InstrRef] Avoid duplicate instruction numbers in x86-lea-fixup

This new-ish LEA-fixup code path creates two substitutions for an
instruction number -- this is incorrect because each Value should be
replaced by a single replacement Value. Fix by deleting the duplicate
substitution. Add some test coverage for this path with debug-info
attached.

Differential Revision: https://reviews.llvm.org/D119232