[DAGCombiner] eliminate shuffle of insert element

I noticed this pattern in D38316 / D38388. We failed to combine a shuffle that is either 
repeating a scalar insertion at the same position in a vector or translated to a different 
element index.

Like the earlier patch, this could be an instcombine too, but since we opted to make this 
a DAG transform earlier, I've made this one a DAG patch too.

We do not need any legality checking because the new insert is identical to the existing 
insert except that it may have a different constant insertion operand.

The constant insertion test in test/CodeGen/X86/vector-shuffle-combining.ll was the 
motivation for D38756.

Differential Revision: https://reviews.llvm.org/D40209

llvm-svn: 320050