[mips] Weaken asm predicate for memory offsets

The isMemWithSimmOffset predicate rejects relocations which is incorrect
behaviour. Linkers and other tools should handle|warn|error when the
field overflows.

Reviewers: dsanders, vkalintiris

Differential Revision: http://reviews.llvm.org/D20727

llvm-svn: 270995
16 files changed