optimizer: eliminate more isdefined checks

[aviatesk]

Follows up #44708 -- in that PR I missed the most obvious optimization
opportunity, i.e. we can safely eliminate isdefined checks when all
fields are defined at allocation site.
This change allows us to eliminate capturing closure constructions when
the body and callsite of capture closure is available within a optimized
frame, e.g.:

function abmult(r::Int, x0)
    if r < 0
        r = -r
    end
    f = x -> x * r
    return @inline f(x0)
end

diff --git a/_master.jl b/_pr.jl
index ea06d865b75..c38f221090f 100644
--- a/_master.jl
+++ b/_pr.jl
@@ -1,24 +1,19 @@
 julia> @code_typed abmult(-3, 3)
 CodeInfo(
-1 ── %1  = Core.Box::Type{Core.Box}
-│    %2  = %new(%1, r@_2)::Core.Box
-│    %3  = Core.isdefined(%2, :contents)::Bool
-└───       goto #3 if not %3
+1 ──       goto #3 if not true
 2 ──       goto #4
 3 ──       $(Expr(:throw_undef_if_not, :r, false))::Any
-4 ┄─ %7  = (r@_2 < 0)::Any
-└───       goto #9 if not %7
-5 ── %9  = Core.isdefined(%2, :contents)::Bool
-└───       goto #7 if not %9
+4 ┄─ %4  = (r@_2 < 0)::Any
+└───       goto #9 if not %4
+5 ──       goto #7 if not true
 6 ──       goto #8
 7 ──       $(Expr(:throw_undef_if_not, :r, false))::Any
-8 ┄─ %13 = -r@_2::Any
-9 ┄─ %14 = φ (#4 => r@_2, #8 => %13)::Any
-│    %15 = Core.isdefined(%2, :contents)::Bool
-└───       goto #11 if not %15
+8 ┄─ %9  = -r@_2::Any
+9 ┄─ %10 = φ (#4 => r@_2, #8 => %9)::Any
+└───       goto #11 if not true
 10 ─       goto #12
 11 ─       $(Expr(:throw_undef_if_not, :r, false))::Any
-12 ┄ %19 = (x0 * %14)::Any
+12 ┄ %14 = (x0 * %10)::Any
 └───       goto #13
-13 ─       return %19
+13 ─       return %14
 ) => Any