[vectorizer] Fix a trivial oversight where we always requested the

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200211 91177308-0d34-0410-b5e6-96231b3b80d8