This patch updates the cost model to cost intrinsics that return
multiple values (in structs) correctly. Previously, the cost model only
thought intrinsics that return `VectorType` need scalarizing, which
meant it cost intrinsics that return multiple vectors (that need
scalarizing) way too cheap (giving it the cost of a single function
call).
This patch also adds a custom cost for llvm.sincos when a vector
function library is available, as certain VFs can be expanded (later in
code gen) to a vector function, reducing the cost to a single call (+
the possible loads from the vector function returns values via output
pointers).
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