[GlobalISel] NFC: Add some test coverage for s158

158 = 32 * 5 - 2

This is a wide type which may benefit from a different widening scheme than
types which are multiples of 64. For example, if 32-bit and 64-bit scalars
are both allowed, and a type is a multiple of 32, or is closer to a multiple
of 32, it *may* be better to

- Widen to the wide multiple of 32
- Break up the type into 32-bit chunks

Anyway, we don't have any test coverage for this at all, so for the sake of
making sure we test it, let's add some test coverage.