* Implement expression type conversion for constant values

* Fix a problem setting a name on a constant value that died because no symbol table was passed in
* Add some comments describing the passes
* Implement a new peephole:
     // Peephole optimize the following instructions:
     // %t = cast <T1>* %P to <T2> * ;; If T1 is losslessly convertable to T2
     // store <T2> %V, <T2>* %t
     //
     // Into:
     // %t = cast <T2> %V to <T1>
     // store <T1> %t2, <T1>* %P

llvm-svn: 1080