[vectorizer] Fix a trivial oversight where we always requested the number of vector registers rather than toggling between vector and scalar register number based on VF. I don't have a test case as I spotted this by inspection and on X86 it only makes a difference if your target is lacking SSE and thus has *no* vector registers.
If someone wants to add a test case for this for ARM or somewhere else
where this is more significant, that would be awesome.
Also made the variable name a bit more sensible while I'm here.