Make OffsetArray maximum! not as type unstable

The change in #50429 moves around some dispatch boundaries and pushes
the allocations in the offsetarrays `maximum!` test over the limit.
The implementation of that code is massively type unstable. Somewhat,
ironically, the whole original point of that test was to test that
the implementation was not type-unstable (#28941), so actually opt our
OffsetArrays implementation into the interface that's supposed to
guarantee that.

If this PR is fine here, I'll submit the same upstream to avoid
diverging the implementations too much.

test/offsetarray.jl

@@ -627,15 +627,15 @@ end
     B = OffsetArray(reshape(1:24, 4, 3, 2), -5, 6, -7)
     for R in (fill(0, -4:-1), fill(0, -4:-1, 7:7), fill(0, -4:-1, 7:7, -6:-6))
         @test @inferred(maximum!(R, B)) == reshape(maximum(B, dims=(2,3)), axes(R)) == reshape(21:24, axes(R))
-        @test @allocated(maximum!(R, B)) <= 800
+        @test @allocated(maximum!(R, B)) <= 300
         @test @inferred(minimum!(R, B)) == reshape(minimum(B, dims=(2,3)), axes(R)) == reshape(1:4, axes(R))
-        @test @allocated(minimum!(R, B)) <= 800
+        @test @allocated(minimum!(R, B)) <= 300
     end
     for R in (fill(0, -4:-4, 7:9), fill(0, -4:-4, 7:9, -6:-6))
         @test @inferred(maximum!(R, B)) == reshape(maximum(B, dims=(1,3)), axes(R)) == reshape(16:4:24, axes(R))
-        @test @allocated(maximum!(R, B)) <= 800
+        @test @allocated(maximum!(R, B)) <= 300
         @test @inferred(minimum!(R, B)) == reshape(minimum(B, dims=(1,3)), axes(R)) == reshape(1:4:9, axes(R))
-        @test @allocated(minimum!(R, B)) <= 800
+        @test @allocated(minimum!(R, B)) <= 300
     end
     @test_throws DimensionMismatch maximum!(fill(0, -4:-1, 7:7, -6:-6, 1:1), B)
     @test_throws DimensionMismatch minimum!(fill(0, -4:-1, 7:7, -6:-6, 1:1), B)

test/testhelpers/OffsetArrays.jl

@@ -197,6 +197,7 @@ Base.show(io::IO, r::IdOffsetRange) = print(io, IdOffsetRange, "(values=",first(
 
 # Optimizations
 @inline Base.checkindex(::Type{Bool}, inds::IdOffsetRange, i::Real) = Base.checkindex(Bool, inds.parent, i - inds.offset)
+Base._firstslice(r::IdOffsetRange) = IdOffsetRange(Base._firstslice(r.parent), r.offset)
 
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 # origin.jl