Fix PR2434. When scanning for exising binary operator to reuse don't

Fix PR2434. When scanning for exising binary operator to reuse don't
take into account the instrucion pointed by InsertPt. Thanks to it,
returning the new value of InsertPt to the InsertBinop() caller can be
avoided. The bug was, actually, in visitAddRecExpr() method which wasn't
correctly handling changes of InsertPt. There shouldn't be any
performance regression, as -gvn pass (run after -indvars) removes any
redundant binops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52291 91177308-0d34-0410-b5e6-96231b3b80d8