LowerTypeTests: Fix quadratic complexity.
Currently we have quadratic complexity in LowerTypeTests because
ScopedSaveAliaseesAndUsed loops over all aliases for each disjoint
set, and the number of aliases and number of disjoint sets is
roughly proportional to the program size. Fix that by moving
ScopedSaveAliaseesAndUsed to LowerTypeTestsModule::lower() so that
we do this only once.
Reviewers: fmayer, vitalybuka
Reviewed By: vitalybuka
Pull Request: https://github.com/llvm/llvm-project/pull/135875
