[VPlan] Determine Vector Width programmatically.

With this change, the VPlan native path is triggered with the directive:

   #pragma clang loop vectorize(enable)

There is no need to specify the vectorize_width(N) clause.

Patch by Francesco Petrogalli <francesco.petrogalli@arm.com>

Differential Revision: https://reviews.llvm.org/D57598

llvm-svn: 357156