[InstSimplify] Simplify select i1 ConstExpr, i1 true, i1 false to ConstExpr

`select i1 non-const, i1 true, i1 false` has been optimized to
`non-const`. There is no reason that we can not optimize `select i1
ConstExpr, i1 true, i1 false` to `ConstExpr`.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D151631