[TableGen] Remove some copies around PatternToMatch.

Summary:
This patch does a few things that should remove some copies around PatternsToMatch. These were noticed while reviewing code for D34341.

Change constructor to take Dstregs by value and move it into the class. Change one of the callers to add std::move to the argument so that it gets moved.

Make AddPatternToMatch take PatternToMatch by rvalue reference so we can move it into the PatternsToMatch vector. I believe we should have a implicit default move constructor available on PatternToMatch. I chose rvalue reference because both callers call it with temporaries already.

Reviewers: RKSimon, aymanmus, spatel

Reviewed By: aymanmus

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34411

llvm-svn: 306251