[VPLAN] Minor improvement to testing and debug messages.

1. Use computed VF for stress testing.
2. If the computed VF does not produce vector code (VF smaller than 2), force VF to be 4.
3. Test vectorization of i64 data on AArch64 to make sure we generate VF != 4 (on X86 that was already tested on AVX).

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

Differential Revision: https://reviews.llvm.org/D59952

llvm-svn: 358056