Reviewed by Maciej Stachowiak & Darin Adler.

        Improve string concatenation (as coded in JS as a sequence of adds).

        Detect patterns corresponding to string concatenation, and change the bytecode
        generation to emit a new op_strcat instruction.  By handling the full set of
        additions within a single function we do not need allocate JSString wrappers
        for intermediate results, and we can calculate the size of the output string
        prior to allocating storage, in order to prevent reallocation of the buffer.

        1.5%-2% progression on Sunspider, largely due to a 30% progression on date-format-xparb.

        * bytecode/CodeBlock.cpp:
        (JSC::CodeBlock::dump):
            Add new opcodes.
        * bytecode/Opcode.h:
            Add new opcodes.
        * bytecompiler/BytecodeGenerator.cpp:
        (JSC::BytecodeGenerator::emitStrcat):
        (JSC::BytecodeGenerator::emitToPrimitive):
            Add generation of new opcodes.
        * bytecompiler/BytecodeGenerator.h:
            Add generation of new opcodes.
        * interpreter/Interpreter.cpp:
        (JSC::Interpreter::privateExecute):
            Add implmentation of new opcodes.
        * jit/JIT.cpp:
        (JSC::JIT::privateCompileMainPass):
        (JSC::JIT::privateCompileSlowCases):
            Add implmentation of new opcodes.
        * jit/JITStubs.cpp:
        (JSC::JITStubs::cti_op_to_primitive):
        (JSC::JITStubs::cti_op_strcat):
            Add implmentation of new opcodes.
        * jit/JITStubs.h:
            Add implmentation of new opcodes.
        * parser/Nodes.cpp:
        (JSC::BinaryOpNode::emitStrcat):
        (JSC::BinaryOpNode::emitBytecode):
        (JSC::ReadModifyResolveNode::emitBytecode):
            Add generation of new opcodes.
        * parser/Nodes.h:
        (JSC::ExpressionNode::):
        (JSC::AddNode::):
            Add methods to allow identification of add nodes.
        * parser/ResultType.h:
        (JSC::ResultType::definitelyIsString):
        (JSC::ResultType::forAdd):
            Fix error in detection of adds that will produce string results.
        * runtime/Operations.h:
        (JSC::concatenateStrings):
            Add implmentation of new opcodes.
        * runtime/UString.cpp:
        (JSC::UString::appendNumeric):
            Add methods to append numbers to an existing string.
        * runtime/UString.h:
        (JSC::UString::Rep::createEmptyBuffer):
        (JSC::UString::BaseString::BaseString):
            Add support for creating an empty string with a non-zero capacity available in the BaseString.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@43331 268f45cc-cd09-0410-ab3c-d52691b4dbfc