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

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

Summary:
Previously, MCSymbolRefExpr::create() was called with a StringRef of the symbol
name, which it would then search for in the Symbols StringMap (from MCContext).

However, relative labels (which are temporary symbols) are apparently not stored
in the Symbols StringMap, so we end up creating a new {$,.L}tmp symbol
({$,.L}tmp00, {$,.L}tmp10 etc.) each time we create an MCSymbolRefExpr by
passing in the symbol name as a StringRef.

Fortunately, there is a version of MCSymbolRefExpr::create() which takes an
MCSymbol* and we already have an MCSymbol* at that point, so we can just pass
that in instead of the StringRef.

I also removed the local StringRef calls to MCSymbolRefExpr::create() from
expandMemInst(), as those cases can be handled by evaluateRelocExpr() anyway.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9938

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239897 91177308-0d34-0410-b5e6-96231b3b80d8