Reland "[InstCombine] Fold nested selects"

The change was reverted because one of the changes were suspected of
causing a miscompile, but said miscompile was (confirmed to be) fixed
before the revert happened, by 07ecdd9b.

https://alive2.llvm.org/ce/z/GjCXkB
https://alive2.llvm.org/ce/z/Guz2tt

Fixes https://github.com/llvm/llvm-project/issues/59393

This reverts commit d73383c1,
and relands commmit 9ddff66d.