[mips][ias] Handle N64 compound relocations and R_MIPS_SUB in needsRelocateWithSymbol()

Summary:
This eliminates the default case for N64 that was left out of r269047.

The change to R_MIPS_SUB is needed in this patch to make this testable since
%lo(%neg(%gp_rel(foo))) and %hi(%neg(%gp_rel(foo))) remain the only ways to get
a compound relocation from the assembler.

Reviewers: sdardis, rafael

Subscribers: dsanders, llvm-commits, sdardis

Differential Revision: http://reviews.llvm.org/D20097

llvm-svn: 269280