[InstrProf] Add option to disable loop opt after PGO

Add the `-enable-post-pgo-loop-rotation` option to enable or disable the loop rotation transformation [1]. With some instrumentations, e.g., function entry coverage [2], loop rotation is not necessary and can lead to some surprise differences in codegen, even for functions where instrumentation is blocked with `noprofile` or `skipprofile`. The default value is `true` so the default behavior does not change.

[1] https://www.llvm.org/docs/LoopTerminology.html#loop-terminology-loop-rotate
[2] https://reviews.llvm.org/D116180

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D131817