[TableGen] Use vector::append instead of looping and calling push_back. NFC
[TableGen] Use vector::append instead of looping and calling push_back. NFC

Both vectors contain unsigned so we can just use append to do the copying. Not only is this shorter, but it should be able to predict the final size and only grow the vector once if needed.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@328033 91177308-0d34-0410-b5e6-96231b3b80d8

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