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

[aheejin]

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?

[dschuff]

yeah, seems fine. we can always change our mind in the future even in the absence of a TODO :)

[dschuff]

Just noticed, but the discussion should maybe have gone into a comment on Phab rather than the commit message itself...

llvm/trunk/include/llvm/IR/IntrinsicsWebAssembly.td

 // Since wasm does not use landingpad instructions, these instructions return
 // exception pointer and selector values until we lower them in WasmEHPrepare.
 def int_wasm_get_exception : Intrinsic<[llvm_ptr_ty], [llvm_token_ty],
                                        [IntrHasSideEffects]>;
 def int_wasm_get_ehselector : Intrinsic<[llvm_i32_ty], [llvm_token_ty],
                                         [IntrHasSideEffects]>;
 // This is the same as llvm.wasm.get.exception except that it does not take a
 // token operand. This is only for instruction selection purpose.
-// TODO Remove this redundant intrinsic and do custom lowering on
-// int_wasm_get_exception instead
 def int_wasm_extract_exception : Intrinsic<[llvm_ptr_ty], [],
                                            [IntrHasSideEffects]>;
 
 // WebAssembly EH must maintain the landingpads in the order assigned to them
 // by WasmEHPrepare pass to generate landingpad table in EHStreamer. This is
 // used in order to give them the indices in WasmEHPrepare.
 def int_wasm_landingpad_index: Intrinsic<[], [llvm_token_ty, llvm_i32_ty],
                                          [IntrNoMem]>;