TableGen/CodeGenDAGPatterns: addPredicateFn only once

Summary:
The predicate function is added in InlinePatternFragments, no need to
do it here. As a result, all uses of addPredicateFn are located in
InlinePatternFragments.

Test confirmed that there are no changes to generated files when
building all (non-experimental) targets.

Change-Id: I720e42e045ca596eb0aa339fb61adf6fe71034d5

Reviewers: arsenm, rampitec, RKSimon, craig.topper, hfinkel, uweigand

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D51993

llvm-svn: 343977