Optimize the deletion of equivalent expressions that are invalidated by variables going out of scope.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#1035

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In order to fix issue #1017, variables going out of scope were tracked and equivalent expressions that were invalidated by such variables were removed. But this causes the compile time to increase by 2.35%. Therefore, the deletion of invalid equivalent expressions needs to be optimized. (Ref PR: checkedc/checkedc-clang#1031)

[secure-sw-dev-bot]

Comment from @dtarditi:

The change in CFG construction to include lifetime ends of variables may have affected the compile-time too. That needs to be considered as part of the compile-time investigation. When we first started using the CFG for checking bounds declarations, I remember constructing the CFG being reasonably expensive in terms of its effect on compile time (i.e. increasing compile-time by at least a few percent).

[secure-sw-dev-bot]

Comment from @sulekhark:

Compile time measurements summed up over all clang and checkedc test cases (averaged over six runs):

No CFGLifetimeEnds elements emitted in the CFG and no removal of invalidated equivalent expressions : 6496 s ---A
CFGLifetimeEnds elements emitted in the CFG but no removal of invalidated equivalent expressions : 6597 s ---B
CFGLifetimeEnds elements emitted in the CFG and removal of invalidated equivalent expressions : 6664 s ---C

A -> B : 1.55% increase
B -> C : 1.02% increase
A -> C : 2.59% increase