Rollup merge of #64386 - tspiteri:const-abs2, r=oli-obk

Rollup merge of #64386 - tspiteri:const-abs2, r=oli-obk

use `sign` variable in abs and wrapping_abs methods

This also makes the code easier to understand by hinting at the significance of `self >> ($BITS - 1)`.

Also, now `overflowing_abs` simply uses `wrapping_abs`, which is clearer and avoids a potential performance regression in the LLVM IR.

This PR follows from the discussion from #63786.

r? @eddyb
cc @nikic