The patch turns on epilogue unroll for loops with constant recurency start.

The patch turns on epilogue unroll for loops with constant recurency start.
Summary:

Set unroll remainder to epilog if a loop contains a phi with constant parameter:

  loop:
  pn = phi [Const, PreHeader], [pn.next, Latch]
  ...

Reviewer: hfinkel

Differential Revision: http://reviews.llvm.org/D27004

From: Evgeny Stupachenko <evstupac@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296770 91177308-0d34-0410-b5e6-96231b3b80d8