Revert D84057 "[PGO][PGSO] Remove a temporary flag used for gradual rollout."

This reverts commit e64afefd. It caused
a PGO bootstrapped clang to crash on many source files.

`__llvm_profile_instrument_range` seems to trigger a null pointer dereference.

Call stack:
__llvm_profile_instrument_range
llvm::APInt::udiv(llvm::APInt const&) const
getRangeForAffineARHelper