[mips] Lift the assertion on the types that can be used with MipsGPRel

Post commit review of rL308619 highlighted the need for handling N64
with -fno-pic. Testing reveale a stale assert when generating a GP
relative addressing mode.

This patch removes that assert and adds the necessary patterns for
MIPS64 to perform gp relative addressing with -fno-pic
(and the implicit -mno-abicalls + -mgpopt).

Reviewers: atanasyan, nitesh.jain

Differential Revision: https://reviews.llvm.org/D36472

llvm-svn: 310713