make _unsetindex fast for isbits eltype (#56364)

fixes
#56359 (comment)
```
using Plots

function f(n)
    a = Vector{Int}(undef, n)
    s = time_ns()
    resize!(a, 8)
    time_ns() - s
end

x = 8:10:1000000
y = f.(x)

plot(x, y)
```

![image](https://github.com/user-attachments/assets/5a1fb963-7d44-4cac-bedd-6f0733d4cf56)

(cherry picked from commit e802eff)

Author: oscardssmith

base/genericmemory.jl

@@ -80,17 +80,16 @@ function _unsetindex!(A::MemoryRef{T}) where T
     MemT = typeof(mem)
     arrayelem = datatype_arrayelem(MemT)
     elsz = datatype_layoutsize(MemT)
-    isboxed = 1; isunion = 2
+    isbits = 0; isboxed = 1; isunion = 2
+    arrayelem == isbits && datatype_pointerfree(T::DataType) && return A
     t = @_gc_preserve_begin mem
     p = Ptr{Ptr{Cvoid}}(@inbounds pointer(A))
     if arrayelem == isboxed
         Intrinsics.atomic_pointerset(p, C_NULL, :monotonic)
     elseif arrayelem != isunion
-        if !datatype_pointerfree(T::DataType)
-            for j = 1:Core.sizeof(Ptr{Cvoid}):elsz
-                # XXX: this violates memory ordering, since it writes more than one C_NULL to each
-                Intrinsics.atomic_pointerset(p + j - 1, C_NULL, :monotonic)
-            end
+        for j = 1:Core.sizeof(Ptr{Cvoid}):elsz
+            # XXX: this violates memory ordering, since it writes more than one C_NULL to each
+            Intrinsics.atomic_pointerset(p + j - 1, C_NULL, :monotonic)
         end
     end
     @_gc_preserve_end t