Rewrite `StructLiteralExp::toElem` to store individual fields instead of generating a constant to fill the entire struct with a single `store`. This is much more efficient at compile time (fixing #320) and vastly reduces the size of the emitted code. Since LLVM no longer needs to keep the data for all fields in "registers" until the store happens, it should also be more efficient at run time in cases where the fields aren't assigned with constants. There's also some code clean-up by removing duplicated logic.

// Copyright (c) 1999-2002 by Digital Mars
// All Rights Reserved
// written by Walter Bright
// www.digitalmars.com

struct GCStats
{
    size_t poolsize;		// total size of pool
    size_t usedsize;		// bytes allocated
    size_t freeblocks;		// number of blocks marked FREE
    size_t freelistsize;		// total of memory on free lists
    size_t pageblocks;		// number of blocks marked PAGE
}