Reapply [Dominators] Add findNearestCommonDominator() for Instructions (NFC)

Reapply with checks for instructions in unreachable blocks. A test
case for this was added in 1ee4a93b.

-----

This is a recurring pattern: We want to find the nearest common
dominator (instruction) for two instructions, but currently only
provide an API for the nearest common dominator of two basic blocks.

Add an overload that accepts and return instructions.