fix some type instabilities introduced by #39607 (#41094)

After #39607, `Iterators.peel` can return `nothing` and the return type
can be more inaccurate than before for certain iterative inputs where
inference doesn't understand its length, e.g.
`Iterators.peel(::<:AbstractString)`.

The following JET analysis shows an example of this type instability:
```julia
═════ 2 possible errors found ═════
┌ @ coreio.jl:4 Base.println(Core.tuple(Core.typeassert(Base.stdout, Base.IO)), xs...)
│┌ @ strings/io.jl:73 Base.print(Core.tuple(io), xs, Core.tuple("\n")...)
││┌ @ strings/io.jl:46 Base.print(io, x)
│││┌ @ show.jl:1283 Base.show_unquoted(Base.IOContext(io, Base.=>(:unquote_fallback, false)), ex, 0, -1)
││││┌ @ show.jl:1634 Base.show_unquoted_quote_expr(io, Base.getproperty(ex, :value), indent, prec, 0)
│││││┌ @ show.jl:1651 Base.isidentifier(sym)
││││││┌ @ show.jl:1339 Base.isidentifier(Base.string(s))
│││││││┌ @ show.jl:1335 Base.indexed_iterate(Base.Iterators.peel(s), 1)
││││││││┌ @ tuple.jl:92 Base.iterate(I)
│││││││││ no matching method found for call signature (Tuple{typeof(iterate), Nothing}): Base.iterate(I::Nothing)
││││││││└───────────────
│││││││┌ @ show.jl:1335 Base.indexed_iterate(Base.Iterators.peel(s), 2, _3)
││││││││┌ @ tuple.jl:97 Base.iterate(I, state)
│││││││││ no matching method found for call signature (Tuple{typeof(iterate), Nothing, Int64}): Base.iterate(I::Nothing, state::Int64)
││││││││└───────────────
Core.Const(nothing)
```

This PR adds code refactors or manual annotations and seems to fix
some of regressions within Base.

base/compiler/ssair/domtree.jl

@@ -593,7 +593,7 @@ function naive_idoms(blocks::Vector{BasicBlock})
             if isempty(blocks[n].preds)
                 continue
             end
-            firstp, rest = Iterators.peel(Iterators.filter(p->p != 0, blocks[n].preds))
+            firstp, rest = Iterators.peel(Iterators.filter(p->p != 0, blocks[n].preds))::NTuple{2,Any}
             new_doms = copy(dominators[firstp])
             for p in rest
                 intersect!(new_doms, dominators[p])

base/io.jl

@@ -885,8 +885,9 @@ end
 
 function readuntil(io::IO, target::AbstractString; keep::Bool=false)
     # small-string target optimizations
-    isempty(target) && return ""
-    c, rest = Iterators.peel(target)
+    x = Iterators.peel(target)
+    isnothing(x) && return ""
+    c, rest = x
     if isempty(rest) && c <= '\x7f'
         return readuntil_string(io, c % UInt8, keep)
     end

base/show.jl

@@ -1330,9 +1330,10 @@ julia> Meta.isidentifier(:x), Meta.isidentifier("1x")
 ```
 """
 function isidentifier(s::AbstractString)
-    isempty(s) && return false
+    x = Iterators.peel(s)
+    isnothing(x) && return false
     (s == "true" || s == "false") && return false
-    c, rest = Iterators.peel(s)
+    c, rest = x
     is_id_start_char(c) || return false
     return all(is_id_char, rest)
 end

base/strings/search.jl

@@ -162,11 +162,12 @@ in(c::AbstractChar, s::AbstractString) = (findfirst(isequal(c),s)!==nothing)
 function _searchindex(s::Union{AbstractString,ByteArray},
                       t::Union{AbstractString,AbstractChar,Int8,UInt8},
                       i::Integer)
-    if isempty(t)
+    x = Iterators.peel(t)
+    if isnothing(x)
         return 1 <= i <= nextind(s,lastindex(s))::Int ? i :
                throw(BoundsError(s, i))
     end
-    t1, trest = Iterators.peel(t)
+    t1, trest = x
     while true
         i = findnext(isequal(t1),s,i)
         if i === nothing return 0 end
@@ -420,7 +421,7 @@ function _rsearchindex(s::AbstractString,
         return 1 <= i <= nextind(s, lastindex(s))::Int ? i :
                throw(BoundsError(s, i))
     end
-    t1, trest = Iterators.peel(Iterators.reverse(t))
+    t1, trest = Iterators.peel(Iterators.reverse(t))::NTuple{2,Any}
     while true
         i = findprev(isequal(t1), s, i)
         i === nothing && return 0