Fix indexing in _mapreducedim for OffsetArrays (#55506)
The destination array was being indexed incorrectly if it had offset
indices. This led to the following on nightly:
```julia
julia> using OffsetArrays
julia> r = 5:100;
julia> a = OffsetVector(r, 2);
julia> sum(a, dims=1)
1-element OffsetArray(::Vector{Int64}, 3:3) with eltype Int64 with indices 3:3:
0
julia> sum(a)
5040
```
The indexing was marked `@inbounds`, so this was not throwing an error.
This PR also follows #55329 and only marks the indexing operations as
`@inbounds`, omitting the function calls.
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Co-authored-by: Matt Bauman <mbauman@juliahub.com>
