[mips][ias] R_MIPS_(GOT|HI|LO|PC)16 and R_MIPS_GPREL32 do not need symbols.

Summary:
In theory, care must be taken to ensure that pairs of R_MIPS_(GOT|HI|LO)16
make the same decision on both relocs in the reloc pair but in practice
this isn't as hard as it sounds and only limits the complexity of the
predicate used. We handle all three with the same code to ensure their
decisions always agree with each other.

Reviewers: sdardis

Subscribers: rafael, dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D19016

llvm-svn: 268900