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.

llvm-svn: 52291