[FuncSpec] Move invariant computation for spec cost out of loop (NFC-ish)

Noticed that the computation for function specialization cost of a
function wouldn't change during the traversal of the arguments for the
function. We could hoist the computation out of the traversal. I
observed about ~1% improvement on compile time for spec2017. But I guess
it may not be precise. This should be NFC and fine.

Reviewed By: Sjoerd Meijer

Differential Revision: https://reviews.llvm.org/D107621