Relax constraints on inlining for some single calls

[ianatol]

Fixes #43104

I'm not sure if this is the correct condition, but it seems to pass all relevant tests. My first real look into the inlining logic, so I could certainly be missing something here.

CC @aviatesk

[aviatesk]

We can also handle constant-prop'ed callsites.
If we implement the similar enhancement within maybe_handle_const_call! as well, I think we can also pass the following test case:

@inline isGoodType2(cnd, @nospecialize x::Type) =
    x !== Any && !(@noinline (cnd ? Core.Compiler.isType : Base.has_free_typevars)(x))
let # aggressive inlining of single, abstract method match (with constant-prop'ed reuslt)
    src = code_typed((Type, Any,)) do x, y
        isGoodType2(true, x), isGoodType2(true, y)
    end |> only |> first
    # both callsites should be inlined (with constant-prop'ed result)
    @test count(isinvoke(:isType), src.code) == 2
    @test count(isinvoke(:has_free_typevars), src.code) == 0
    # `isGoodType2(y::Any)` isn't fully convered, thus a runtime type check and fallback dynamic dispatch should be inserted
    @test count(iscall((src,isGoodType2)), src.code) == 1
end

[vtjnash]

Isn't fully_covered already simply a field of MethodMatch?

[aviatesk]

ah, yeah I agree that we can just use it for the single, non-isdispatchtuple case.

[aviatesk]

Rebased and added the enhancement for constant-prop' handling case too.
I think this is ready for merge.

[aviatesk]

This seems to certainly eliminate some runtime dispatches:

before

    JULIA usr/lib/julia/sys-o.a
Generating REPL precompile statements... 36/36
Executing precompile statements... 1451/1486
Precompilation complete. Summary:
Total ─────── 115.295423 seconds
Generation ──  88.625775 seconds 76.8684%
Execution ───  26.669648 seconds 23.1316%

after

    JULIA usr/lib/julia/sys-o.a
Generating REPL precompile statements... 36/36
Executing precompile statements... 1424/1459
Precompilation complete. Summary:
Total ─────── 113.570618 seconds
Generation ──  86.702678 seconds 76.3425%
Execution ───  26.867939 seconds 23.6575%