[DAGCombiner] Remove explicit call to AddToWorklist in sqrt and reciprocal computations

Summary: These nodes end up being processed regardless due to DAGCombiner ensuring arguments are processed. This changes the order in which nodes are processed, which fixes an issue on PowerPC.

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri, mcberg2017, stefanp, hfinkel

Subscribers: nemanjai, MaskRay, jsji, steven.zhang, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66548

llvm-svn: 369662