[IR] `IRBuilderBase::CreateSelect()`: if cond is a constant i1, short-circuit

While we could emit such a tautological `select`,
it will stick around until the next instsimplify invocation,
which may happen after we count the cost of this redundant `select`.
Which is precisely what happens with loop vectorization legality checks,
and that artificially increases the cost of said checks,
which is bad.

There is prior art for this in `IRBuilderBase::CreateAnd()`/`IRBuilderBase::CreateOr()`.

Refs. https://reviews.llvm.org/D109368#3089809