Remove more superfluous .str() and replace std::string concatenation with Twine.

Following r233392, http://llvm.org/viewvc/llvm-project?rev=233392&view=rev.

llvm-svn: 233555
diff --git a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
index c24805a6..93c6ea0 100644
--- a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -2571,7 +2571,7 @@
   SmallString<16> Tmp;
   Tmp += Base;
   Tmp += ' ';
-  Op.setTokenValue(Tmp.str());
+  Op.setTokenValue(Tmp);
 
   // If this instruction starts with an 'f', then it is a floating point stack
   // instruction.  These come in up to three forms for 32-bit, 64-bit, and