Changed the fundemental architecture of Operands for Instructions. Now

Changed the fundemental architecture of Operands for Instructions.  Now
Operands are maintained as a vector<Use> in the User class, and operator
iterators are provided as before.  Getting an operand no longer requires
a virtual function call.

WARNING: getOperand(x) where x >= getNumOperands() will now assert instead
of returning null!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149 91177308-0d34-0410-b5e6-96231b3b80d8