[SimpleLoopUnswitch] partial unswitch needs to be careful when replacing invariants with constants

When partial unswitch operates on multiple conditions at once, .e.g:
   if (Cond1 || Cond2 || NonInv) ...

it should infer (and replace) values for individual conditions only on one
side of unswitch and not another.

More precisely only these derivations hold true:
   (Cond1 || Cond2) == false  =>  Cond1 == Cond2 == false
   (Cond1 && Cond2) == true   =>  Cond1 == Cond2 == true

By the way we organize unswitching it means only replacing on "continue" blocks
and never on "unswitched" ones. Since trivial unswitch does not have "unswitched"
blocks it does not have this problem.

Fixes PR 39568.

Reviewers: chandlerc, asbirlea
Differential Revision: https://reviews.llvm.org/D54211

llvm-svn: 346350