[InstCombine] Simplify a known nonzero incoming value of PHI

Summary:
When a PHI is used only to be compared with zero, it is possible to replace an
incoming value with any non-zero constant if the incoming value can be proved as
a known nonzero value. For example, in below code, we can replace the incoming value %v with
any non-zero constant based on the fact that the PHI is only used to be compared with zero
and %v is a known non-zero value:
  %v = select %cond, 1, 2
  %p = phi [%v, BB] ...
  %c = icmp eq, %p, 0

Reviewers: mcrosier, jmolloy, sanjoy

Subscribers: hfinkel, mcrosier, majnemer, llvm-commits, haicheng, bmakam, mssimpso, gberry

Differential Revision: http://reviews.llvm.org/D16240

llvm-svn: 260530