[TargetLowering] Deduplicate choosing InlineAsm constraint between ISels (#67057)

Given a list of constraints for InlineAsm (ex. "imr") I'm looking to
modify the order in which they are chosen. Before doing so, I noticed a
fair
amount of logic is duplicated between SelectionDAGISel and GlobalISel
for this.

That is because SelectionDAGISel is also trying to lower immediates
during selection. If we detangle these concerns into:
1. choose the preferred constraint
2. attempt to lower that constraint

Then we can slide down the list of constraints until we find one that
can be lowered. That allows the implementation to be shared between
instruction selection frameworks.

This makes it so that later I might only need to adjust the priority of
constraints in one place, and have both selectors behave the same.