iommu_gas: Drop needless bound check in lowermatch

The loop iteration in iommu_gas_lowermatch checks the bound
a->common->lowaddr twice per loop iteration. Rewrite to test only once
per iteration. Do not worry about passing to iommu_gas_match_one a
range wholly beyond lowaddr. Since that function checks the upper end
of the range against lowaddr, it'll get rejected there.

Reviewed by: alc
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35522