[ARM] Fixed an assumption of power-of-2 vector MVT

I am about to introduce some non-power-of-2 width vector MVTs. This

commit fixes a power-of-2 assumption that my forthcoming change would

otherwise break, as shown by test/CodeGen/ARM/vcvt_combine.ll and

vdiv_combine.ll.

Differential Revision: https://reviews.llvm.org/D58927

Change-Id: I56a282e365d3874ab0621e5bdef98a612f702317