Selects of the form `cond ? 1 : 0` are created during unrolling of
setcc+vselect. Currently these are not optimized away post-legalization
even if fully redundant. Having these extra selects sitting between
things can prevent other folds from applying.
Enabling this requires some mitigations in the ARM backend, in
particular in the interaction with MVE support. There's two changes
here:
* Form CSINV/CSNEG/CSINC from CMOV, rather than only creating it during
SELECT_CC lowering. (After this change, the lowering in SELECT_CC can be
dropped without test changes, let me know if I should do that.)
* Support pushing negations through CMOV in more cases, in particular if
the operands are constant or the negation can be handled by flipping
lshr/ashr.
Additionally, in the X86 backend, try to simplify CMOV to SETCC if only the
low bit is demanded.
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