[WebAssembly] Don't override default implementation of isOffsetFoldingLegal. NFC.

The default implementation does we want and is going to more compatible
with dynamic linking (-fPIC) support that is planned.

This is NFC because currently we only build wasm with
`-relocation-model=static` which in turn means that the default
`isOffsetFoldingLegal` always returns true today.

Differential Revision: https://reviews.llvm.org/D54661

llvm-svn: 356410