[TTI] Treat AND/OR with widenable conditions as free of cost

Because widenable conditions with eventually lower into a constant, such instructions
as `and`, `or` etc. will also be optimized away. Treat them as free.

This is an important thing to have if we want that guards represented as experimental.guard
calls and in their explicit form (branch by `and` with widenable condition) have the same cost
for unroller and other passes like this.

Differential Revision: https://reviews.llvm.org/D146034
Reviewed By: nikic