* Fixes#2358. Added to the reducer the ability to remove a function that is not directly called. Factored out some code from the optimizer to help with this.
Fixes#2120
Enhanced the reducer so that it can merge blocks together, leveraging the functionality extracted from the block_merge pass in the optimizer.
* Add OperandToUndefReductionPass.
Fixes#2115.
Also added some tests that are similar to those in OperandToConstantReductionPassTest.
In addition, refactor FindOrCreateGlobalUndef into reduction_util.cpp. Fixes#2184.
Removed many documentation comments that were identical or very similar to the overridden function's documentation comment.
Add a spirv-reduce pass which removes OpName and OpMemberName instructions.
This is useful to enable other reduction passes, e.g. RemoveUnreferencedInstruction may not be able to remove an instruction creating an id whose only usage is an OpName for this id.
* Added a reduction pass to replace ids with ids of the same type that dominate them.
* Introduce helper method for querying whether an operand type is an input id.
