[Bug target/125907] Missed Optimization (x86 bit-test `bt`)

[liuhongt at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125907

--- Comment #5 from Hongtao Liu <liuhongt at gcc dot gnu.org> ---

> The initilization of 0 is very cheap, not sure if it's worth to do uncprop
> for it.

Or maybe we should prevent propgate a single-use SSA-NAME into a PHI since it
extends the live range plus hurt backend optimization for cmp.