MinVecRegSize is currently hardcoded to 128; this patch adds a cl::opt to allow
changing it. I tried not to change any existing behavior for the default case.
This option would be handy for writing smaller tests. Also, currently it's not
even possible to write a test for vectorization of i1/i2/i4 stores, as we only
look for at most 16 accesses, while the minimal total size should be 128bits.
It's possible to write such test using phi-nodes, but that looks like a bug (I
plan to follow up with a fix for it too).