The vector combiner will process all instructions as it first loops
through the function, adding any newly added and deleted instructions to
a worklist which is then processed when all nodes are done. These leaves
extra uses in the graph as the initial processing is performed, leading
to sub-optimal decisions being made for other combines. This changes it
so that trivially dead instructions are removed immediately. The main
changes that this requires is to make sure iterator invalidation does not
occur.
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