Fix complexity of bounds of nested pure intrinsics

[abadams]

Alternative to #8687

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Fixes #8686
Fixes #8750 - a drive-by fix to restore the behavior of debug(0)

[vksnk]

Thanks for the fix!

[alexreinking]

It would be nice for these sorts of failures to be caught by clang-tidy:

/Users/halidenightly/build_bot/worker/halide-testbranch-main-llvm20-arm-64-osx-make/halide-source/test/correctness/bounds_of_pure_intrinsics.cpp:18:13: error: variable 'ops' set but not used [-Werror,-Wunused-but-set-variable]
        int ops = 0;
            ^
1 error generated.