cmd/compile: prove misscompilation in slicemask folding leaves garbage in the upper half of the 32bits of the register when slicing a slice by a non constant value that is `< cap` [1.25 backport]

[gopherbot]

@Jorropo requested issue #79874 to be considered for backport to the next 1.25 minor release.

cc @gopherbot please open backport issues for 1.26 and 1.25

This a misscompilation that can lead to incorrect behavior.