[InstSimplify] fold identity shuffles (recursing if needed)

This patch simplifies the examples from D31509 and D31927 (PR30630) and catches 
the basic identity shuffle tests that Zvi recently added.

I'm not sure if we have something like this in DAGCombiner, but we should?

It's worth noting that "MaxRecurse / RecursionLimit" is only 3 on entry at the moment. 
We might want to bump that up if there are longer shuffle chains like this in the wild.

For now, we're ignoring shuffles that have undef mask elements because it's not
clear how those should be handled.

Differential Revision: https://reviews.llvm.org/D31960

llvm-svn: 300714