tweaks for inter-procedural constraint back-prop'

note that the changes for `isnothing` and `ismissing` aren't necessary,
but they reduce the number of method definitions for good reason;
the less the number of methods they have, the better we can
back-propagate type constraints, because even after a package defines
their own new methods for them we can keep to use our `InterConditional`
logic as far as far as the number of methods is
[≤3](https://github.com/JuliaLang/julia/blob/5c6e21edbfd8f0c7d16ea01c91d1c75c30d4eaa1/base/compiler/types.jl#L119)

base/essentials.jl

@@ -817,8 +817,7 @@ const missing = Missing()
 
 Indicate whether `x` is [`missing`](@ref).
 """
-ismissing(::Any) = false
-ismissing(::Missing) = true
+ismissing(x) = x === missing
 
 function popfirst! end
 

base/show.jl

@@ -1382,7 +1382,7 @@ function operator_associativity(s::Symbol)
 end
 
 is_expr(@nospecialize(ex), head::Symbol)         = isa(ex, Expr) && (ex.head === head)
-is_expr(@nospecialize(ex), head::Symbol, n::Int) = is_expr(ex, head) && length((ex::Expr).args) == n
+is_expr(@nospecialize(ex), head::Symbol, n::Int) = is_expr(ex, head) && length(ex.args) == n
 
 is_quoted(ex)            = false
 is_quoted(ex::QuoteNode) = true

base/some.jl

@@ -65,8 +65,7 @@ Return `true` if `x === nothing`, and return `false` if not.
 !!! compat "Julia 1.1"
     This function requires at least Julia 1.1.
 """
-isnothing(::Any) = false
-isnothing(::Nothing) = true
+isnothing(x) = x === nothing
 
 
 """