[MVE] Don't try to unroll vectorised MVE loops

Due to the nature of the beat system in the MVE architecture, along with tail
predication and low-overhead loops, unrolling has less benefit compared to
normal loops. You can not, for example, hide the latency of a load with other
instructions as you can for scalar code. Preventing unrolling also makes the
code easier to read and reason about.

So if a loop contains vector code, don't enable the runtime unrolling. At least
for the time being.

Differential Revision: https://reviews.llvm.org/D65803

llvm-svn: 368530