Alternative instructions in the Linux kernel may modify control flow in
a function. As such, it is unsafe to optimize functions with alternative
instructions until we properly support CFG alternatives.
Previously, we marked functions with alt instructions before the
emission, but that could be too late if we remove or replace
instructions with alternatives. We could have marked functions as
non-simple immediately after reading .altinstructions, but it's nice to
be able to view functions after CFG is built. Thus assign the non-simple
status after building CFG.
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