[DebugInfo] LiveDebugValues: locations with different exprs should not be merged

When comparing variable locations, LiveDebugValues currently considers only
the machine location, ignoring any DIExpression applied to it. This is a
problem because that DIExpression can do pretty much anything to the machine
location, for example dereferencing it.

This patch adds DIExpressions to that comparison; now variables based on the
same register/memory-location but with different expressions will compare
differently, and be dropped if we attempt to merge them between blocks. This
reduces variable coverage-range a little, but only because we were producing
broken locations.

Differential Revision: https://reviews.llvm.org/D66942

llvm-svn: 370877