[VectorCombine] add a debug flag to skip all transforms

As suggested in D75145 -

I'm not sure why, but several passes have this kind of disable/enable flag
implemented at the pass manager level. But that means we have to duplicate
the flag for both pass managers and add code to check the flag every time
the pass appears in the pipeline.

We want a debug option to see if this pass is misbehaving regardless of the
pass managers, so just add a disablement check at the single point before
any transforms run.

Differential Revision: https://reviews.llvm.org/D75204