[llvm-dev] RFC: Stop using redundant PHI node entries for multi-edge predecessors

[Chandler Carruth via llvm-dev]

Today, the IR requires that if you have multiple edges from A to B
(typically with a switch) any phi nodes in B must have an equal number of
entries for A, but that all of them must have the same value.

This seems rather annoying....
1) It creates multiple uses of values in A for no apparently good reason
2) It makes updating PHI nodes using sets of predecessors incredibly hard
3) There is no correspondence between the edges and the PHI node entries
other than number, making most of the code that does handle this very
ad-hoc "count off N repetitions of the same thing" style code, which is
brittle and hard to test effectively.
4) I suspect bugs are quite common given that PHINode::removeIncomingValue
accepts a basic block but only removes the first incoming value, leaving
the others in place.

I can't see any serious use case for this symmetry either. These aren't
uses (and can't be), and there doesn't seem to be any lost functionality if
we only have a single entry.

It also seems likely to open up more efficient in-memory representations if
it is possible at some point to build maps of these things.

Thoughts? It appears entirely possible to change the verifier to check that
we don't have duplicates, and to read bitcode with duplicates and merge
them so that there is no backwards compatibility issue.

-Chandler
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