[ValueTracking] Enable -branch-on-poison-as-ub by default

Now that SimpleLoopUnswitch and other transforms no longer introduce
branch on poison, enable the -branch-on-poison-as-ub option by
default. The practical impact of this is mostly better flag
preservation in SCEV, and some freeze instructions no longer being
necessary.

Differential Revision: https://reviews.llvm.org/D125299