[SimplifyCFG] Need to remove useless comparison after turning switch to icmp

[wweiandrew]

For example,

define i64 @demo(i64 %x) {
entry:
    switch i64 %x, label %bb3 [
        i64 0, label %bb1
        i64 1, label %bb2
    ]
bb1:
    ret i64 0
bb2:
    %0 = icmp eq i64 %x, 100 ; this will necessarily be false because %x == 1
    br i1 %0, label %bb4, label %bb5
bb3:
    unreachable
bb4:
    ret i64 200
bb5:
    ret i64 10
}

See: https://godbolt.org/z/3nobrzoKa
This is a known issue from rust: rust-lang/rust#85133 (comment)

We want to get the IR result after SimplifyCFG：

define i64 @src(i64 %x) {
%entry:
  %switch = icmp eq i64 %x, 0
  %. = select i1 %switch, i64 0, i64 10
  ret i64 %.
}