[SimpleLoopUnswitch] Add freeze if branch execs for partial unswitching.

We cannot skip the freezing the condition if the unswitched branch
executes, if the condition is a chain of ANDs/ORs. For example, if if we
have an AND %c1, %c2 with  %c1 == undef and %c2 == 0, there would be no
branch on undef in the original code, but a branch on undef if we
unswitch %c1.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D124603