[Bug tree-optimization/82255] Vectorizer cost model overcounts cost of some vectorized loads

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82255

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yeah, this is a known "missing feature" in the cost model.  It's pessimistic
about the fancy way we can process both strided SLP loads and stores.

I didn't want to tackle this before trying to sort out the ugliness in SLP
vs. non-SLP costing (that we need two copies of all the cost compute in two
different places).  I wanted to add sth like
STMT_VINFO_{PROLOGUE,BODY}_COST_VEC
and SLP_NODE_{PROLOGUE,BODY}_COST_VEC that can be filled from vectorizable_*
and just collected from the cost processing code.