spirv-reduce: add OperandToUndefReductionPass (#2200)

* 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.