[LoopVectorizer] fix test file to not run the entire -O3 pipeline

This test file has a long history of edits from changes outside
of vectorization, and it would happen again with the proposal in
D61726.

End-to-end testing shouldn't be happening in a test file that is
specifically checking for vector masked load/store ops.
Larger-scale testing goes in PhaseOrdering or the test-suite.

I've hopefully preserved the intent by taking what was completely
unoptimized IR in some tests and passing that through the -O1
pipeline. That becomes the input IR, and now we just run the loop
vectorizer and verify that the vector masked ops are produced as
expected.

llvm-svn: 360340