[VectorCombine] Improve shuffle select shuffle-of-shuffles

This in an extension to the code added in D123911 which added vector
combine folding of shuffle-select patterns, attempting to reduce the
total amount of shuffling required in patterns like:
  %x = shuffle %i1, %i2
  %y = shuffle %i1, %i2
  %a = binop %x, %y
  %b = binop %x, %y
  shuffle %a, %b, selectmask

This patch extends the handing of shuffles that are dependent on one
another, which can arise from the SLP vectorizer, as-in:
  %x = shuffle %i1, %i2
  %y = shuffle %x

The input shuffles can also be emitted, in which case they are treated
like identity shuffles. This patch also attempts to calculate a better
ordering of input shuffles, which can help getting lower cost input
shuffles, pushing complex shuffles further down the tree.

Differential Revision: https://reviews.llvm.org/D128732