Reapply with checks for instructions in unreachable blocks. A test
case for this was added in 1ee4a93b15bb.
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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.
This reverts commit 7f0de9573f758f5f9108795850337a5acbd17eef.
This is missing handling for !isReachableFromEntry() blocks, which
may be relevant for some callers. Revert for now.
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.