Improve implementation of ParserATNSimulator::getConflictingAlts() #1060

Job	Run time
cpp-lib-build (macos-12, clang, ON)	4m 15s
cpp-lib-build (macos-12, clang, OFF)	3m 41s
build (macos-12, tool)	2m 0s
cpp-lib-build (macos-12, gcc, ON)	4m 4s
build (macos-12, cpp)	15m 55s
cpp-lib-build (macos-12, gcc, OFF)	3m 40s
build (macos-12, csharp)	5m 29s
cpp-lib-build (ubuntu-20.04, clang, ON)	3m 22s
build (macos-12, dart)	3m 50s
cpp-lib-build (ubuntu-20.04, clang, OFF)	3m 30s
build (macos-12, go)	3m 2s
cpp-lib-build (ubuntu-20.04, gcc, ON)	4m 29s
build (macos-12, java)	2m 5s
cpp-lib-build (ubuntu-20.04, gcc, OFF)	5m 8s
build (macos-12, javascript)	2m 9s
cpp-lib-build (windows-2022, clang, ON)	4m 27s
build (macos-12, typescript)	11m 52s
cpp-lib-build (windows-2022, clang, OFF)	5m 56s
build (macos-12, php)	2m 45s
cpp-lib-build (windows-2022, cl)	4m 31s
build (macos-12, python3)	2m 3s
build (ubuntu-20.04, tool)	1m 6s
build (ubuntu-20.04, cpp)	17m 37s
build (ubuntu-20.04, csharp)	5m 1s
build (ubuntu-20.04, dart)	3m 41s
build (ubuntu-20.04, go)	2m 19s
build (ubuntu-20.04, java)	1m 4s
build (ubuntu-20.04, javascript)	1m 31s
build (ubuntu-20.04, typescript)	9m 51s
build (ubuntu-20.04, php)	1m 22s
build (ubuntu-20.04, python3)	1m 7s
build (windows-2022, tool)	1m 37s
build (windows-2022, cpp)	22m 3s
build (windows-2022, csharp)	7m 9s
build (windows-2022, dart)	4m 14s
build (windows-2022, go)	6m 12s
build (windows-2022, java)	1m 59s
build (windows-2022, javascript)	2m 19s
build (windows-2022, typescript)	20m 59s
build (windows-2022, php)	1m 55s
build (windows-2022, python3)	1m 47s
	3h 33m 6s

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