[GVNHoist] Change the key for VNtoInsns to a pair

While debugging GVNHoist, I found it confusing that the entries in a
VNtoInsns were not always value numbers.  They _usually_ were except for
StoreInst in which case they were a hash of two different value numbers.

This leads to two observations:
- It is more difficult to debug things when the semantic contents of
  VNtoInsns changes over time.
- Using a single value number is not much cheaper, the value of
  VNtoInsns is a SmallVector.
- It is not immediately clear what the algorithm would do if there were
  hash collisions in the StoreInst case.

Using a DenseMap of std::pair sidesteps all of this.

N.B.  The changes in the test were due their sensitivity to the
iteration order of VNtoInsns which has changed.

llvm-svn: 275761