[PruneEH] Don't split musttail call from ret

[JosephTremoulet]

Summary

When inserting an unreachable after a noreturn call, we must ensure
that it's not a musttail call to avoid breaking the IR invariants for
musttail calls.

[majnemer]

LGTM

llvm/lib/Transforms/IPO/PruneEH.cpp

if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
           DeleteBasicBlock(UnwindBlock, CG);  // Delete the new BB.
 
         ++NumRemoved;
         MadeChange = true;
       }
 
     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; )
       if (CallInst *CI = dyn_cast<CallInst>(I++))
-        if (CI->doesNotReturn() && !isa<UnreachableInst>(I)) {
+        if (CI->doesNotReturn() && !CI->isMustTailCall() &&
+            !isa<UnreachableInst>(I)) {
           // This call calls a function that cannot return.  Insert an
           // unreachable instruction after it and simplify the code.  Do this
           // by splitting the BB, adding the unreachable, then deleting the
           // new BB.
           BasicBlock *New = BB->splitBasicBlock(I);
 
           // Remove the uncond branch and add an unreachable.
           BB->getInstList().pop_back();

llvm/test/Transforms/PruneEH/musttail.ll

+; RUN: opt -prune-eh -S < %s | FileCheck %s
+
+declare void @noreturn()
+
+define void @testfn() {
+    ; A musttail call must be followed by (optional bitcast then) ret,
+    ; so make sure we don't insert an unreachable
+    ; CHECK: musttail call void @noreturn
+    ; CHECK-NOT: unreachable
+    ; CHECK-NEXT: ret void
+    musttail call void @noreturn() #0
+    ret void
+}
+
+attributes #0 = { noreturn }