delegates to the regular x86-32 convention which handles byval, but only
after it handles a few cases, and it's necessary to handle byval before
handling those cases. This fixes PR3122 (and rdar://6400815), llvm-gcc
miscompiling LLVM.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60453 91177308-0d34-0410-b5e6-96231b3b80d8
