[BPI] Avoid ConstantExpr::get()

Use ConstantFoldBinaryOpOperands() instead, to prepare for the case
where not all binary operators have a constant expression form.

I believe this code actually intended to set OnlyIfReduced=true,
however ConstantExpr::get() actually accepts a Flags argument at
that position (and OnlyIfReducedTy as the next argument), so this
ended up creating a constant expression with some random flag
(probably exact or nuw depending on which).