[TableGen] Use emplace_back to add to PatternsToMatch in GenerateVariants. Use...

[TableGen] Use emplace_back to add to PatternsToMatch in GenerateVariants. Use std::move when adding to PatternsToMatch in AddPatternToMatch.

We already used emplace_back in at least one other place so be
consistent.

AddPatternToMatch already took PTM as an rvalue reference, but
we need to use std::move again to move it into the PatternToMatch
vector.