[DAGCombine] Move setcc of freeze fold to brcond

This fold goes against the usual approach of pushing freeze into
operands. The idea behind the fold is that if the setcc feeds into
a brcond, the freeze can be dropped entirely.

Move the fold to brcond, where we can remove the freeze directly.
This ensures that there can be no infinite combine loops due to
conflicting transforms.

Differential Revision: https://reviews.llvm.org/D152544