[TargetLowering] move "o" and "X" constraint handling to base class

These constraints are machine agnostic; there's no reason to handle
these per-arch. If arches don't support these constraints, then they
will fail elsewhere during instruction selection. We don't need virtual
calls to look these up; TargetLowering::getInlineAsmMemConstraint should
only be overridden by architectures with additional unique memory
constraints.

Reviewed By: echristo, MaskRay

Differential Revision: https://reviews.llvm.org/D100416