[mips] Changed the way symbols are handled in dla and la instructions to allow simple expressions.

Summary:
An instruction like "(d)la $5, symbol+8" previously would have crashed the
assembler as it contains an expression. This is now fixed.
A few tests cases have also been changed to reflect these changes, however
these should only be syntax changes. Some new test cases have also been
added.

Patch by Scott Egerton.

Reviewers: vkalintiris, dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12760

llvm-svn: 249311