[SimpleLoopUnswitch] Freeze individual OR/AND operands.

In some cases, it is not enough to freeze the final AND/OR operation
when chaining a number of invariant conditions together.

After creating a chain of ANDs/ORs, we assume all unswitched operands to
be either true or false. But if any of the operands is poison, the rest
of the operands could have any value after branching on the frozen
condition.

To avoid that, freeze individual operands, if needed. In some cases this
may lead to unnecessary freezes, but it seems required at least for some
cases (see trivial-unswitch-freeze-individual-conditions.ll)

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D124554