[SimplifyCFG] Tail-merge calls with sideeffects

This was deliberately disabled during my rewrite of SinkIfThenToEnd to keep behaviour
at least vaguely consistent with the previous version and keep it as close to NFC as
I could.

There's no real reason not to merge sideeffect calls though, so let's do it! Small fixup
along the way to ensure we don't create indirect calls.

Should fix PR28964.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280215 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1433,11 +1433,6 @@ static bool canSinkLastInstruction(ArrayRef<BasicBlock*> Blocks,
     if (isa<PHINode>(I) || I->isEHPad() || isa<AllocaInst>(I) ||
         I->getType()->isTokenTy())
       return false;
-    // Apart from loads and stores, we won't move anything that could
-    // change memory or have sideeffects.
-    if (!isa<StoreInst>(I) && !isa<LoadInst>(I) &&
-        (I->mayHaveSideEffects() || I->mayHaveSideEffects()))
-      return false;
     // Everything must have only one use too, apart from stores which
     // have no uses.
     if (!isa<StoreInst>(I) && !I->hasOneUse())
@@ -1472,10 +1467,11 @@ static bool canSinkLastInstruction(ArrayRef<BasicBlock*> Blocks,
       if (!canReplaceOperandWithVariable(I0, OI))
         // We can't create a PHI from this GEP.
         return false;
-      if ((isa<CallInst>(I0) || isa<InvokeInst>(I0)) && OI != 0)
-        // Don't create indirect calls!
+      // Don't create indirect calls! The called value is the final operand.
+      if ((isa<CallInst>(I0) || isa<InvokeInst>(I0)) && OI == OE - 1) {
         // FIXME: if the call was *already* indirect, we should do this.
         return false;
+      }
       ++NumPHIsRequired;
     }
   }

test/Transforms/SimplifyCFG/sink-common-code.ll

@@ -344,6 +344,29 @@ if.end:
 ; CHECK-NOT: load
 ; CHECK-NOT: store
 
+; The call should be commoned.
+define i32 @test13a(i1 zeroext %flag, i32 %w, i32 %x, i32 %y) {
+entry:
+  br i1 %flag, label %if.then, label %if.else
+
+if.then:
+  %sv1 = call i32 @bar(i32 %x)
+  br label %if.end
+
+if.else:
+  %sv2 = call i32 @bar(i32 %y)
+  br label %if.end
+
+if.end:
+  %p = phi i32 [ %sv1, %if.then ], [ %sv2, %if.else ]
+  ret i32 1
+}
+declare i32 @bar(i32)
+
+; CHECK-LABEL: test13a
+; CHECK: %[[x:.*]] = select i1 %flag
+; CHECK: call i32 @bar(i32 %[[x]])
+
 ; CHECK: !0 = !{!1, !1, i64 0}
 ; CHECK: !1 = !{!"float", !2}
-; CHECK: !2 = !{!"an example type tree"}
+; CHECK: !2 = !{!"an example type tree"}