[llvm] [VPlan] Factor first-lane-scalar logic in VPI::execute (NFC) (PR #196207)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> > Are you able to get any compile time numbers on llvm-compile-time-tracker?
> 
> Just [noise](https://llvm-compile-time-tracker.com/compare.php?from=2fd8d41d7b8824637be32d569c1e1482128b057b&to=d51511b27c1ad9d49017aadb9c6adc41426c026b&stat=instructions:u), I think. The motivation for the patch is to use IsSingleScalar in more places in follow ups?

More context: I think this patch will finally allow us to make progress on consolidating WidenCast and WidenGEP (we already have a scalar-GEP VPI in https://github.com/llvm/llvm-project/pull/193510).

https://github.com/llvm/llvm-project/pull/196207