[llvm][PPCISelDAGToDAG] rename ppc-codegen to ppc-isel

[nickdesaulniers]

Summary

Every other subclass of SelectionDAGISel calls this pass "<arch>-isel".
No existing tests refer to ppc-codegen so this is purely a cosmetic
change to bring the pass name in line with other architecture's
SelectionDAGISel subclasses.

[MaskRay]

Looks great! The consistency with other targets is good.

[MaskRay]

(CC #PowerPC to inform potential users)

llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

 #include <limits>
 #include <memory>
 #include <new>
 #include <tuple>
 #include <utility>
 
 using namespace llvm;
 
-#define DEBUG_TYPE "ppc-codegen"
+#define DEBUG_TYPE "ppc-isel"
 #define PASS_NAME "PowerPC DAG->DAG Pattern Instruction Selection"
 
 STATISTIC(NumSextSetcc,
           "Number of (sext(setcc)) nodes expanded into GPR sequence.");
 STATISTIC(NumZextSetcc,
           "Number of (zext(setcc)) nodes expanded into GPR sequence.");
 STATISTIC(SignExtensionsAdded,
           "Number of sign extensions for compare inputs added.");