There were a lot of unnecessary copying and resizing and sloppiness in the use of BitVector for liveness analysis.  This tries to reduce the amount of copying and associated memory allocation.

Also, works around a ConstantRelocatable hashing problem that was causing a huge slowdown in MINIMAL mode for the vector_shuffle scons test.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1746613002 .