[InstCombine] Missed optimization for `zext(trunc nuw(x))`

[Il-Capitano]

Godbolt link: https://godbolt.org/z/Ws1jPz4rE

Instcombine transforms this function:

define i64 @test(i64 %n) {
    %trunc = trunc nuw i64 %n to i32
    %zext = zext i32 %trunc to i64
    ret i64 %zext
}

into this:

define i64 @test(i64 %n) {
  %zext = and i64 %n, 4294967295
  ret i64 %zext
}

However, because of the nuw flag on trunc, this could be simplified into this instead:

define i64 @test(i64 %n) {
    ret i64 %n
}

[nikic]

See #88609.