[JumpThreading] Unfold selects that depend on the same condition

Summary:
These are good candidates for jump threading. This enables later opts
(such as InstCombine) to combine instructions from the selects with
instructions out of the selects. SimplifyCFG will fold the select
again if unfolding wasn't worth it.

Patch by James Molloy and Pablo Barrio.

Reviewers: rengolin, haicheng, sebpop

Subscribers: jojo, jmolloy, llvm-commits

Differential Revision: https://reviews.llvm.org/D26391

llvm-svn: 286236