Introduces Ice::SmallBitVector.

Modifies ConstantRelocatables so that known offsets (i.e.,
not offsets to the code stream) do no require GlobalContext
allocations.

Modifies Cfg-local containers to use the CfgLocalAllocator.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1738443002 .