[mips] [IAS] Allow .set assignments for already defined symbols.

Summary:
This is not possible when using the IAS for MIPS, but it is possible when using the IAS for other architectures and when using GAS for MIPS.


Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8578

llvm-svn: 234316