[Vectorizer] Convert index-select to comprehensive arg(min|max) tests.

Generalize and extend the tests in index-select.cpp to cover
argmin/argmax with both finding the first and last indices.

The test cases cover a range of predicates (<, <=, >, >=) with a range
of initial values, trip counts and types.

This adds extensive test coverage for
llvm/llvm-project#141431 and follow-ups.

Author: fhahn