Round up the size of byval arguments to MinAlign

Otherwise we can end up with an argument frame size that is not a
multiple of stack slot size, which is very awkward.

This fixes PR20547, which was a bug in x86_64 Sys V vararg handling.
However, it's much easier to test this with x86 callee-cleanup
functions, which previously ended in "retl $6" instead of "retl $8".

This does affect behavior of all backends, but it presumably fixes the
same bug in all of them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214980 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CallingConvLower.cpp

@@ -52,6 +52,7 @@ void CCState::HandleByVal(unsigned ValNo, MVT ValVT,
     Align = MinAlign;
   MF.getFrameInfo()->ensureMaxAlignment(Align);
   TM.getSubtargetImpl()->getTargetLowering()->HandleByVal(this, Size, Align);
+  Size = unsigned(RoundUpToAlignment(Size, MinAlign));
   unsigned Offset = AllocateStack(Size, Align);
   addLoc(CCValAssign::getMem(ValNo, ValVT, Offset, LocVT, LocInfo));
 }