Compiler: abstract calls: type assert to help stability

It seems `la` is getting inferred as `Integer` instead of as `Int` for
some reason.

This reduces the number of invalidations from 2061 to 2046 on running
this code:

```julia
struct I <: Integer end
Base.Int64(::I) = 7
```

Author: nsajko

Compiler/src/abstractinterpretation.jl

@@ -1956,7 +1956,7 @@ end
 function abstract_call_builtin(interp::AbstractInterpreter, f::Builtin, (; fargs, argtypes)::ArgInfo,
                                sv::AbsIntState)
     @nospecialize f
-    la = length(argtypes)
+    la = length(argtypes)::Int
     𝕃ᵢ = typeinf_lattice(interp)
     ⊑, ⊏, ⊔, ⊓ = partialorder(𝕃ᵢ), strictpartialorder(𝕃ᵢ), join(𝕃ᵢ), meet(𝕃ᵢ)
     if has_conditional(𝕃ᵢ, sv) && f === Core.ifelse && fargs isa Vector{Any} && la == 4
@@ -2622,7 +2622,7 @@ function abstract_call_known(interp::AbstractInterpreter, @nospecialize(f),
         arginfo::ArgInfo, si::StmtInfo, sv::AbsIntState,
         max_methods::Int = get_max_methods(interp, f, sv))
     (; fargs, argtypes) = arginfo
-    la = length(argtypes)
+    la = length(argtypes)::Int
     𝕃ᵢ = typeinf_lattice(interp)
     if isa(f, Builtin)
         if f === _apply_iterate