Simplify _isequal, _isless, __inc, and __dec

...by removing unnecessary type checks/assertions. This basically
restores the function bodies to what they looked like prior to #40594
and only keeps the changed signatures (with the additional changes to
circumvent #39088..

base/multidimensional.jl

@@ -127,9 +127,7 @@ module IteratorsMD
     @inline isless(I1::CartesianIndex{N}, I2::CartesianIndex{N}) where {N} = _isless(0, I1.I, I2.I)
     @inline function _isless(ret, I1::Tuple{Int,Vararg{Int}}, I2::Tuple{Int,Vararg{Int}})
         newret = ifelse(ret==0, icmp(last(I1), last(I2)), ret)
-        t1, t2 = Base.front(I1), Base.front(I2)
-        # avoid dynamic dispatch by telling the compiler relational invariants
-        return isa(t1, Tuple{}) ? _isless(newret, (), t2::Tuple{}) : _isless(newret, t1, t2::Tuple{Int,Vararg{Int}})
+        return _isless(newret, Base.front(I1), Base.front(I2))
     end
     _isless(ret, ::Tuple{}, ::Tuple{}) = ifelse(ret==1, true, false)
     icmp(a, b) = ifelse(isless(a,b), 1, ifelse(a==b, 0, -1))
@@ -439,9 +437,7 @@ module IteratorsMD
         if __is_valid_range(I, rng) && state[1] != last(rng)
             return true, (I, tail(state)...)
         end
-        t1, t2 = tail(state), tail(indices)
-        # avoid dynamic dispatch by telling the compiler relational invariants
-        valid, I = isa(t1, Tuple{Int}) ? __inc(t1, t2::Tuple{OrdinalRangeInt}) : __inc(t1, t2::Tuple{OrdinalRangeInt,OrdinalRangeInt,Vararg{OrdinalRangeInt}})
+        valid, I = __inc(tail(state), tail(indices))
         return valid, (first(rng), I...)
     end
 
@@ -552,9 +548,7 @@ module IteratorsMD
         if __is_valid_range(I, rng) && state[1] != first(rng)
             return true, (I, tail(state)...)
         end
-        t1, t2 = tail(state), tail(indices)
-        # avoid dynamic dispatch by telling the compiler relational invariants
-        valid, I = isa(t1, Tuple{Int}) ? __dec(t1, t2::Tuple{OrdinalRangeInt}) : __dec(t1, t2::Tuple{OrdinalRangeInt,OrdinalRangeInt,Vararg{OrdinalRangeInt}})
+        valid, I = __dec(tail(state), tail(indices))
         return valid, (last(rng), I...)
     end
 

base/tuple.jl

@@ -383,10 +383,7 @@ filter(f, t::Tuple) = length(t) < 32 ? filter_rec(f, t) : Tuple(filter(f, collec
 isequal(t1::Tuple, t2::Tuple) = length(t1) == length(t2) && _isequal(t1, t2)
 _isequal(::Tuple{}, ::Tuple{}) = true
 function _isequal(t1::Tuple{Any,Vararg{Any}}, t2::Tuple{Any,Vararg{Any}})
-    isequal(t1[1], t2[1]) || return false
-    t1, t2 = tail(t1), tail(t2)
-    # avoid dynamic dispatch by telling the compiler relational invariants
-    return isa(t1, Tuple{}) ? _isequal((), t2::Tuple{}) : _isequal(t1, t2::Tuple{Any,Vararg{Any}})
+    return isequal(t1[1], t2[1]) && _isequal(tail(t1), tail(t2))
 end
 function _isequal(t1::Any32, t2::Any32)
     for i = 1:length(t1)