Fix poor performance of sameSource predicate #3640

Job	Run time
Prepare CodeQL unit test matrix	8s
Run unit tests (c, 2.19.4, codeql-cli/v2.19.4, codeql-bundle-v2.19.4, ubuntu-latest-xl, codeql-cl...	37m 25s
Run unit tests (cpp, 2.19.4, codeql-cli/v2.19.4, codeql-bundle-v2.19.4, ubuntu-latest-xl, codeql-...	1h 42m 12s
Validate test results	7s
	2h 19m 52s

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