[mips] [IAS] Fix LA with relative label operands.

Summary:
Call MCSymbolRefExpr::create() with a MCSymbol* argument, not with a StringRef
of the Symbol's name, in order to avoid creating invalid temporary symbols for
relative labels (e.g. {$,.L}tmp00, {$,.L}tmp10 etc.).

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10498

llvm-svn: 239901