[mips][ias] Expand on r238751 to cover as many relocs as possible.

Summary:
Relocs that can be converted from absolute to PC-relative now do so if IsPCRel
is true. Relocs that require PC-relative now call llvm_unreachable() if IsPCRel
is false and similarly those that require absolute assert that IsPCRel is false.

Note that while it looks like some relocs (e.g. R_MIPS_26) can be converted into
the MIPS32r6/MIPS64r6 relocs (R_MIPS_PC*_S2), it isn't actually valid to do so.

Placeholders have been left in the testcase for unsupported relocs and relocs
that cannot be generated at the moment.

Reviewers: vkalintiris

Reviewed By: vkalintiris

Subscribers: llvm-commits, rafael

Differential Revision: http://reviews.llvm.org/D10184

llvm-svn: 239817