C++: Divide number of bounds between branches for phi nodes

[paldepind]

This PR changes the heuristic for how the number of bounds is calculated for a guard phi node.

This is an improvement in a few ways:

• It avoids a special case where nodes that are both normal phi nodes and guard phi nodes needed special treatment.

• I think this heuristic makes sense intuitively.

• It fixes a problem where a series of if-else statements with guards on the same variable caused the number of bounds to exponentially increase while the actual range analysis did in fact not introduce any bounds.

  The first commit adds an example of this. In the last commit the change to the expected file around test.c:453 to test.c:463 show how we now handle this case better.

  This fixes a recent problem observed over in coding standards. See: Revert "C++: Accept test changes after github/codeql#21313." codeql-coding-standards#1041.

For reviewing the comment inside nrOfBoundsPhiGuard should help explain what is going on.

[Copilot]

Pull request overview

This PR improves the heuristic for calculating the number of bounds for guard phi nodes in C++ range analysis. The change addresses an exponential growth issue that occurred when analyzing series of if-else statements with guards on the same variable.

Changes:

• Modified nrOfBoundsPhiGuard function to use a new heuristic: (varBounds + 1) / 2 instead of special-casing certain phi nodes
• Added test case repeated_if_else_statements demonstrating the fixed scenario with 11 consecutive if-else statements
• Added helper predicates countNrOfLowerBounds and countNrOfUpperBounds in SimpleRangeAnalysisInternal module
• Updated test query nrOfBounds.ql to output actual bounds counts alongside estimates

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated no comments.

File	Description
cpp/ql/lib/semmle/code/cpp/rangeanalysis/SimpleRangeAnalysis.qll	Changed bounds estimation heuristic in nrOfBoundsPhiGuard from special-casing to dividing by 2; added helper predicates for counting actual bounds
cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/test.c	Added repeated_if_else_statements function demonstrating the exponential growth scenario that is now fixed
cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/nrOfBounds.ql	Enhanced query to also output actual lower and upper bounds counts for validation
cpp/ql/test/library-tests/rangeanalysis/SimpleRangeAnalysis/*.expected	Updated expected test outputs to reflect the improved bounds estimates (line numbers shifted due to new test case)