When using CR bit registers on PPC32, handle the i1 vaarg case

When copying an i1 value into a GPR for a vaarg call, we need to explicitly
zero-extend the i1 value (otherwise an invalid CRBIT -> GPR copy will be
generated).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203041 91177308-0d34-0410-b5e6-96231b3b80d8