[InstCombine] Missed optimization : fold `switch(rol(a, c))`

Alive2 proof: https://alive2.llvm.org/ce/z/uPJXhH

Motivating example

define i64 @src(i64 %a) #0 {
entry:
  %rol = call i64 @llvm.fshl.i64(i64 %a, i64 %a, i64 62)
  switch i64 %rol, label %default [
    i64 0, label %trap.exit
    i64 5, label %trap.exit
  ]

default:
  call void @dummy()
  br label %trap.exit

trap.exit:
  ret i64 0
}

can be folded to:

define i64 @tgt(i64 %a) #0 {
entry:
  switch i64 %a, label %default [
    i64 0, label %trap.exit
    i64 20, label %trap.exit
  ]

default:
  call void @dummy()
  br label %trap.exit

trap.exit:
  ret i64 0
}

Real-world motivation

This snippet of IR is derived from ruby/signal.c@sig_trap (after O3 pipeline). The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, see also:https://godbolt.org/z/E7nz88vbP

Let me know if you can confirm that it's an optimization opportunity, thanks.