[WebAssembly] Fix __clang_call_termiante's argument for foreign exceptions

[aheejin]

Summary

When exceptions are repeatedly thrown in the middle of handling another
exception, we call __clang_call_terminate with the exception pointer
(i32) as an argument. But in case of foreign exceptions, we don't have
the pointer, so we call the function with 0. (This requires
__clang_call_terminate can deal with 0 argument, which will be done
later)

But previously the 0 argument was not added as a i32.const 0 but an
immediate by mistake, causing the call instruction to take not an i32
but rather an exnref, because an exnref is left on top of the value
stack if br_on_exn is not taken.

block i32
  br_on_exn 0, __cpp_exception
                               ;; exnref is on top of stack now
  i32.const 0                  ;; This was missing!
  call __clang_call_terminate
  unreachable
end
call __clang_call_terminate    ;; This takes i32 extracted by br_on_exn

llvm/lib/Target/WebAssembly/WebAssemblyLateEHPrepare.cpp

 //
 // In the destination BB that's the target of br_on_exn, extracted exception
 // values (in C++'s case a single i32, which represents an exception pointer)
 // are placed on top of the wasm stack. Because we can't model wasm stack in
 // LLVM instruction, we use 'extract_exception' pseudo instruction to retrieve
 // it. The pseudo instruction will be deleted later.
 bool WebAssemblyLateEHPrepare::addExceptionExtraction(MachineFunction &MF) {
   const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
+  MachineRegisterInfo &MRI = MF.getRegInfo();
   auto *EHInfo = MF.getWasmEHFuncInfo();
   SmallVector<MachineInstr *, 16> ExtractInstrs;
   SmallVector<MachineInstr *, 8> ToDelete;
   for (auto &MBB : MF) {
     for (auto &MI : MBB) {
       if (MI.getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32) {
         if (MI.getOperand(0).isDead())
           ToDelete.push_back(&MI);
for (auto *Extract : ExtractInstrs) {
     //   call @__clang_call_terminate(%exn)
     //   unreachable
     if (TerminatePads.count(EHPad)) {
       Function *ClangCallTerminateFn =
           MF.getFunction().getParent()->getFunction(
               WebAssembly::ClangCallTerminateFn);
       assert(ClangCallTerminateFn &&
              "There is no __clang_call_terminate() function");
+      unsigned Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
+      BuildMI(ElseMBB, DL, TII.get(WebAssembly::CONST_I32), Reg).addImm(0);
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::CALL_VOID))
           .addGlobalAddress(ClangCallTerminateFn)
-          .addImm(0);
+          .addReg(Reg);
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::UNREACHABLE));
 
     } else {
       BuildMI(ElseMBB, DL, TII.get(WebAssembly::RETHROW)).addReg(ExnReg);
       if (EHInfo->hasEHPadUnwindDest(EHPad))
         ElseMBB->addSuccessor(EHInfo->getEHPadUnwindDest(EHPad));
     }
   }

llvm/test/CodeGen/WebAssembly/exception.ll

 ; CHECK:   try
 ; CHECK:     call      foo
 ; CHECK:   catch
 ; CHECK:     try
 ; CHECK:       call      __cxa_end_catch
 ; CHECK:     catch
 ; CHECK:       block     i32
 ; CHECK:         br_on_exn   0, __cpp_exception
-; CHECK:         call      __clang_call_terminate, 0
+; CHECK:         i32.const  ${{.*}}=, 0
+; CHECK:         call      __clang_call_terminate
 ; CHECK:         unreachable
 ; CHECK:       end_block
 ; CHECK:       call      __clang_call_terminate
 ; CHECK:       unreachable
 ; CHECK:     end_try
 ; CHECK:     rethrow
 ; CHECK:   end_try
 ; CHECK:   call      __cxa_end_catch