[Alignment] TargetLowering::hasPairedLoad must use Align for RequiredAlignment

As per documentation of `hasPairLoad`:
"`RequiredAlignment` gives the minimal alignment constraints that must be met to be able to select this paired load."
In this sense, `0` is strictly equivalent to `1`. We make this obvious by using `Align` instead of unsigned.
There is only one implementor of this interface.

Differential Revision: https://reviews.llvm.org/D82958