[SimplifyCFG] switch on umin can be converted to use default dest

[nikic]

https://alive2.llvm.org/ce/z/_N6nfs

define void @src(i32 %x) {
  %min = call i32 @llvm.umin.i32(i32 %x, i32 3)
  switch i32 %min, label %unreachable [
    i32 0, label %case0
    i32 1, label %case1
    i32 2, label %case2
    i32 3, label %case3
  ]

case0:
   call void @a()
   ret void

case1:
   call void @b()
   ret void

case2:
   call void @c()
   ret void

case3:
   ret void

unreachable:
   unreachable
}

define void @tgt(i32 %x) {
  switch i32 %x, label %case3 [
    i32 0, label %case0
    i32 1, label %case1
    i32 2, label %case2
  ]

case0:
   call void @a()
   ret void

case1:
   call void @b()
   ret void

case2:
   call void @c()
   ret void

case3:
   ret void

unreachable:
   unreachable
}

declare void @a()
declare void @b()
declare void @c() 

The umin here maps "all the remaining cases" to 3 and then has an explicit case for 3. We could use the switch default instead.

(This inverse of this does seem like a reasonable backend lowering strategy though.)