[DFSan] Add efficient fast16labels instrumentation mode.

Adds the -fast-16-labels flag, which enables efficient instrumentation
for DFSan when the user needs <=16 labels.  The instrumentation
eliminates most branches and most calls to __dfsan_union or
__dfsan_union_load.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D84371