[WebAssembly] Be a little more conservative in WebAssemblyFixFunctionBitcasts

We don't have enough information to know if struct types being
bitcast will cause validation failures or not, so be conservative
and allow such cases to persist (fot now).

Fixes: https://bugs.llvm.org/show_bug.cgi?id=38711

Subscribers: dschuff, jgravelle-google, aheejin, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51460

llvm-svn: 341010