C++: Speed up Expr.getFullyConverted slightly
C++: Speed up Expr.getFullyConverted slightly

This change cuts around 15% of the run time off this predicate. It's a
workaround for QL-796.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql