[VPLAN] Minor improvement to testing and debug messages.
- Use computed VF for stress testing.
- If the computed VF does not produce vector code (VF smaller than 2), force VF to be 4.
- 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 <email@example.com>
Differential Revision: https://reviews.llvm.org/D59952