[InlineCost] Simplify constant folding

Use a common ConstantFoldInstOperands-based constant folding
implementation, instead of specifying the folding function for
each function individually. Going through the generic handling
doesn't appear to have any significant compile-time impact.

As the test change shows, this is not NFC, because we now use
DataLayout-aware constant folding, which can do slightly better
in some cases (e.g. those involving GEPs).