[mips][msa] Merge basic_operations_little.ll into basic_operations.ll.

Now that FileCheck supports multiple check prefixes, we don't need to keep the
little and big endian versions of this test separate anymore. Merge them back
together.
[-cxx-abi microsoft] Emit thunks for pointers to virtual member functions

Instead of storing the vtable offset directly in the function pointer and
doing a branch to check for virtualness at each call site, the MS ABI
generates a thunk for calling the function at a specific vtable offset,
and puts that in the function pointer.

This patch adds support for emitting such thunks. However, it doesn't support
pointers to virtual member functions that are variadic, have an incomplete
aggregate return type or parameter, or are overriding a function in a virtual
base class.

Differential Revision: http://llvm-reviews.chandlerc.com/D2104