[WebAssembly] Add new pass to lower int/ptr conversions of reftypes

Add new pass LowerRefTypesIntPtrConv to generate trap
instruction for an inttoptr and ptrtoint of a reference type instead
of erroring, since calling these instructions on non-integral pointers
has been since allowed (see ac81cb7e).

Differential Revision: https://reviews.llvm.org/D107102