[ARM][MVE] Change VCTP operand

The VCTP instruction will calculate the predicate masked based upon
the number of elements that need to be processed. I had inserted the
sub before the vctp intrinsic and supplied it as the operand, but
this is incorrect as the phi should directly feed the vctp. The sub
is calculating the value for the next iteration.

Differential Revision: https://reviews.llvm.org/D67921

llvm-svn: 373188