[ARM][MVE] Fix a corner case of checking for MVE-I with -mfpu=none

-march=armv8.1-m.main+mve.fp+nomve -mfpu=none should disable FP
registers and instructions moving to/from FP registers.

This patch fixes the case when "+mve" (added to the feature list by
"+mve.fp"), is followed by "-mve" (added by "+nomve").

Differential Revision: https://reviews.llvm.org/D72633