[NewPM][opt] Make -enable-new-pm default to LLVM_ENABLE_NEW_PASS_MANAGER

This is controlled by the ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER CMake flag.

https://lists.llvm.org/pipermail/llvm-dev/2021-January/147993.html

Reviewed By: ychen, asbirlea

Differential Revision: https://reviews.llvm.org/D95254