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How do you disable optimization for a function?

I ask because my application often compiles machine-generated code that results in pathological structures that take a long time to optimize, for little benefit. As an example, if a basic block has over a million instructions in it, then DSE can take a while, as it is O(n^2) in the number of instructions in the block.

In my application (at least), this block is typically executed only once at run time, and is not part of a loop, so I really don't care about code quality for that block.

I am able to identify some of these conditions. I would like to tell LLVM to "not try so hard" on the pathological blocks, while keeping optimizations active for ordinary cases.