[CodeGen] Add back setOperationAction/setLoadExtAction/setLibcallName single opcode variants

The work to add ArrayRef helpers (D122557, D123467 etc.) to the TargetLowering::set* methods resulted in all the single opcode calls to these methods being cast to single element ArrayRef on the fly - resulting in a scary >5x increase in build time (identified with vcperf) on MSVC release builds of most of the TargetLowering/ISelLowering files.

This patch adds the back the single opcode variants to various set*Action calls to avoid this issue for now, and updates the ArrayRef helpers to wrap them - I'm still investigating whether the single element ArrayRef build times can be improved.