The Sub instruction cost and the shift-by-zero handling costs (ICmp +
Select) are only needed when the shift amount is non-constant. Move them
inside the `!OpInfoZ.isConstant()` guard to avoid overestimating cost
for constant shift amounts.
The overestimated scalar cost caused SLP vectorizer to incorrectly
prefer vectorizing funnel shifts with constant shift amounts, since SLP
compares vector cost against scalar cost and a falsely high scalar cost
makes vectorization appear more profitable than it actually is.
Fixes#181308.
3ca069c471