Quick: Reduce memory usage and improve compile time.

Move the def-block-matrix from Arena to ScopedArena. Remove
BasicBlockDataFlow::ending_check_v and use a temporary bit
matrix instead. Remove unused BasicBlockDataFlow::phi_v.
Avoid some BitVector::Copy() at the end of null and clinit
check elimination passes when the contents of the source
BitVector is no longer needed.

Change-Id: I8111b2f8a51e63075aa124b528d61b79b6933274