[InstCombine] Remove some uses with replaceUndefsWith() (#89190)

Now that we don't accept undef splat in PatternMatch, we can remove some
uses of replaceUndefsWith(). I believe in all these cases only poison
splats are possible now, in which case no replacement is necessary.

GitOrigin-RevId: eb7ad8853cfe6ede2d592a417a1ec64636a61bb5
11 files changed