[PPCGCodeGeneration] Enable `polly-codegen-perf-monitoring` for PPCGCodegen.

This feature was not enabled for `PPCGCodeGeneration`. Now that this is
enabled, we can benchmark Scops that have been optimised with
`-polly-codegen-ppcg` with the `-polly-codegen-perf-monitoring` option.

Differential Revision: https://reviews.llvm.org/D36934

llvm-svn: 311328