[WebAssembly] Ensure bitcasts that would result in invalid wasm are removed by FixFun...

[WebAssembly] Ensure bitcasts that would result in invalid wasm are removed by FixFunctionBitcasts

Rather than allowing invalid bitcasts to be lowered to wasm
call instructions that won't validate, generate wrappers that
contain unreachable thereby delaying the error until runtime.

Differential Revision: https://reviews.llvm.org/D49517

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338744 91177308-0d34-0410-b5e6-96231b3b80d8