[x32/NaCl] Check if method pointers straddle an eightbyte to classify Hi

Summary:
Currently, with struct my_struct { int x; method_ptr y; };
a call to foo(my_struct s) may end up dropping the last 4 bytes
of the method pointer for x86_64 NaCl and x32.

When checking Has64BitPointers, also check if the method pointer
straddles an eightbyte boundary and classify Hi as well as Lo if needed.

Test Plan: test/CodeGenCXX/x86_64-arguments-nacl-x32.cpp

Reviewers: dschuff, pavel.v.chupin

Subscribers: jfb

Differential Revision: http://reviews.llvm.org/D5555