[PATCH][x86][2/3]: Add -mzero-caller-saved-regs=[skip|used-gpr|all-gpr|used|all]

[Qing Zhao]

Hi,

This is the 2nd patch of the total 3 patches set for providing the new feature -mzero-caller-saved-regs for linux kernel security improvement.

This patch is for resolving the new regressions triggered by the first patch. 

This patch is to Add ix86_any_return_p to check simple_return in a PARALLEL to support:

(jump_insn 39 38 40 5 (parallel [
            (simple_return)
            (unspec [
                    (const_int 0 [0])
                ] UNSPEC_SIMPLE_RETURN)
        ]) "/tmp/x.c":105 -1
     (nil)
 -> simple_return)

Any comments?

Qing

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