[Bug rtl-optimization/106705] constant prop is not undone for switches

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106705

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note the cost for constant loading usually is expensive which is why this case
is usually ignored ...
It is more costly on in-order chips though, especially RISCV where constant
forming takes 3 instructions while on most take 2 (or 1 on some).