PSCEV] Create AddRec for Phis in cases of possible integer overflow,

using runtime checks

Extend the SCEVPredicateRewriter to work a bit harder when it encounters an
UnknownSCEV for a Phi node; Try to build an AddRecurrence also for Phi nodes
whose update chain involves casts that can be ignored under the proper runtime
overflow test. This is one step towards addressing PR30654.

Differential revision: http://reviews.llvm.org/D30041

llvm-svn: 308299