[MachinePipeliner] Fix fragile tests

Currently, some tests are too sensitive to changes in MachinePipeliner.
Some of them are more sensitive to scheduling results than necessary,
others assume the current dependency analysis, which is actually
incorrect. As an example of the former, there is a test that is intended
to verify the correctness of the dependency analysis, but validates the
final scheduling results. These tests can be affected by unrelated
changes in MachinePipeliner. This patch fixes them to make them robust
against such changes.

This patch is a prelude to llvm#121907.

Author: kasuga-fj