RegisterPressure: Simplify close{Top|Bottom}()

- There are no duplicate registers in LiveRegs list we are copying from
  and so we do not need to sort the registers.
- Simply use SmallVector::apend instead of a loop between begin() and end()
  with push_back().

Differential Revision: http://reviews.llvm.org/D12813

llvm-svn: 247588