[WebAssembly] Disable offset folding for function addresses

Wasm does not support function addresses with offsets, but isel can
generate folded SDValues in the form of (@func + offset) without this
patch.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43133.

Reviewed By: dschuff, sbc100

Differential Revision: https://reviews.llvm.org/D107940