[mips] Zero-sized structs cannot be ignored in MipsABIInfo::classifyReturnType() for O32

Summary:
They are returned indirectly which causes the other arguments to move to
the next argument slot.

With this, utils/ABITest does not discover any failing cases in the first
500 attempts on big/little endian for O32. Previously some of these failed.
Also tested N32/N64 little endian (big endian has other known issues) with
no issues.

Reviewers: atanasyan

Reviewed By: atanasyan

Subscribers: atanasyan, cfe-commits

Differential Revision: http://reviews.llvm.org/D4811

llvm-svn: 217147