[SimpleLoopUnswitch] Don't unswitch constant conditions

Added an additional check for constants after simplification of
"select _, true, false" pattern. We need to prevent attempts to unswitch constant
conditions for two reasons:
a) Doing that doesn't make any sense, in the best case it will just burn
some compile time.
b) SimpleLoopUnswitch isn't designed to unswitch constant conditions
(due to (a)), so attempting that can cause miscompiles. The attached
testcase is an example of such miscompile.

Also added an assertion that'll make sure we aren't trying to replace
constants, so it will help us prevent such bugs in future. The assertion
from D110751 is another layer of protection against such cases.

Reviewed By: aeubanks
Differential Revision: https://reviews.llvm.org/D110752