Cherrypick upstream r240600, r240601

  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.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4122
TBR=jvoung@chromium.org
(reviewed upstream)

Review URL: https://codereview.chromium.org/1211623004.

• lib/CodeGen/TargetInfo.cpp[diff]
• test/CodeGenCXX/x86_64-arguments-nacl-x32.cpp[diff]