Fix an assertion in MemCpyOpt pass.

The MemCpyOpt pass is capable of optimizing:
      callee(&S); copy N bytes from S to D.
    into:
      callee(&D);
subject to some legality constraints. 

  Assertion is triggered when the compiler tries to evalute "sizeof(typeof(D))",
while D is an opaque-typed, 'sret' formal argument of function being compiled.
i.e. the signature of the func being compiled is something like this:
  T caller(...,%opaque* noalias nocapture sret %D, ...)

  The fix is that when come across such situation, instead of calling some
utility functions to get the size of D's type (which will crash), we simply
assume D has at least N bytes as implified by the copy-instruction.

rdar://14073661 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183584 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -626,8 +626,10 @@ bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
       return false;
 
     Type *StructTy = cast<PointerType>(A->getType())->getElementType();
-    uint64_t destSize = TD->getTypeAllocSize(StructTy);
-
+    // If StructTy is an opaque type, it should have at least <cpyLen> bytes,
+    // as implified by the copy-instruction.
+    uint64_t destSize = StructTy->isSized() ?
+                        TD->getTypeAllocSize(StructTy) : cpyLen;
     if (destSize < srcSize)
       return false;
   } else {