Narrow `@inbounds` annotations in `accumulate.jl` to only indexing calls (#58200)
`op` should not be `inbounds`-ed here
the test case I added might technically be an illegal (or at least bad)
use of `@propagate_inbounds` in case anyone has a suggestion for a more
natural test case, but nonetheless it demonstrates getting a
`BoundsError` instead of a segfault (worse) or incorrect / junk data
(more worse), both of which happen on master