[WebAssembly] Remove TODO on wasm.extract.exception intrinsic (NFC)

Summary:
We planned to delete this intrinsic and do custom lowering from
`wasm.get.exception`, which has a token argument, to
`EXTRACT_EXCEPTION`, a wasm pseudo instruction that simulates popping a
value from the wasm stack.

To do that, we need to introduce a new `WebAssemblyISD` node for this,
which itself is not a problem, but also have to introduce the
`WebAssemblyISD` namespace in SelectionDAGBuilder.cpp. I don't think any
other targets are doing that in the file. And also putting a
target-specific intrinsic in the common file is a little weird too. (All
other intrinsic functions in this `visitIntrinsicCall` functions are not
target-specific ones. Other target-specific intrinsics are usually
handled in `lib/Target/[TargetName]/[TargetName]ISelLowering.cpp`. The
reason we can't do this is it has a token argument.

Anyway, so I think I prefer the current code with one redundant
intrinsic more than adding one more `WebAssemblyISD` node and
also introducing the `WebAssemblyISD` namespace into
SelectionDAGBuilder.cpp. What do you think?

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D57480

llvm-svn: 352695