[LV] -scalable-vectorization should not override pragmas (#226948)
For loops using #pragma clang loop vectorize(disable), one would expect
vectorisation to be disabled regardless of
-scalable-vectorization=preferred flags.
However, LV used to turn a UserVF=1 (vectorisation disabled) into
UserVF=vscale x 1 when -scalable-vectorization=preferred was passed.