Instead of storing operands as std::vector<Use>, just maintain a pointer

and num operands in the User class.  this allows us to embed the operands
directly in the subclasses if possible.  For example, for binary operators
we store the two operands in the derived class.

The has several effects:
  1. it improves locality because the operands and instruction are together
  2. it makes accesses to operands faster (one less load) if you access them
     through the derived class pointer.  For example this:

Value *GetBinaryOperatorOp(BinaryOperator *I, int i) {
  return I->getOperand(i);
}

Was compiled to:

_Z19GetBinaryOperatorOpPN4llvm14BinaryOperatorEi:
        movl    4(%esp), %edx
        movl    8(%esp), %eax
        sall    $4, %eax
        movl    24(%edx), %ecx
        addl    %ecx, %eax
        movl    (%eax), %eax
        ret

and is now compiled to:

_Z19GetBinaryOperatorOpPN4llvm14BinaryOperatorEi:
        movl    8(%esp), %eax
        movl    4(%esp), %edx
        sall    $4, %eax
        addl    %edx, %eax
        movl    44(%eax), %eax
        ret

Accesses through "Instruction*" are unmodified.

   3. This reduces memory consumption (by about 3%) by eliminating 1 word of
      vector overhead and a malloc header on a seperate object.
   4. This speeds up gccas about 10% (both debug and release builds) on
      large things (such as 176.gcc).  For example, it takes a debug build
      from 172.9 -> 155.6s and a release gccas from 67.7 -> 61.8s

llvm-svn: 19883