Relax assertion in x86_64 byval argument handling for 32-bit pointers

Summary:
Byval argument pair formation assumes that if a type is less than 8 bytes
it must be an integer and not a pointer, which is not true for x32 and NaCl.

Relax the assertion and add a test for a codegen case that triggered it.

Reviewers: jvoung

Subscribers: jfb, cfe-commits

Differential Revision: http://reviews.llvm.org/D10701

llvm-svn: 240600