[LoopUnswitch] Simplify branch condition if it is select with constant operands

This fixes the miscompilation reported in https://reviews.llvm.org/rG5bb38e84d3d0#986154 .

`select _, true, false` matches both m_LogicalAnd and m_LogicalOr, making later
transformations confused.
Simplify the branch condition to not have the form.