[mips] Make symbols an acceptable branch target when expanding...

[mips] Make symbols an acceptable branch target when expanding compare-to-immediate-and-branch macros.

Reviewers: vkalintiris

Subscribers: llvm-commits, vkalintiris, dim, seanbruno, dsanders

Differential Revision: http://reviews.llvm.org/D15369

llvm-svn: 262213