CodeGen: Handle va_start in the entry block

Also fix a small copy-paste bug in X86ISelLowering where Chain should
have been used in place of DAG.getEntryToken().

Fixes PR20828.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216929 91177308-0d34-0410-b5e6-96231b3b80d8