Make isbitstypes use memmove instead of the runtime function in copyto! (#56237)

This might help llvm understand whats going on. Also enzyme really wants
this to look like this to trace through it better.
---------

Co-authored-by: Jameson Nash <vtjnash@gmail.com>

Author: gbaraldi

base/genericmemory.jl

@@ -118,7 +118,17 @@ function unsafe_copyto!(dest::MemoryRef{T}, src::MemoryRef{T}, n) where {T}
     @_terminates_globally_notaskstate_meta
     n == 0 && return dest
     @boundscheck memoryref(dest, n), memoryref(src, n)
-    ccall(:jl_genericmemory_copyto, Cvoid, (Any, Ptr{Cvoid}, Any, Ptr{Cvoid}, Int), dest.mem, dest.ptr_or_offset, src.mem, src.ptr_or_offset, Int(n))
+    if isbitstype(T)
+        tdest = @_gc_preserve_begin dest
+        tsrc = @_gc_preserve_begin src
+        pdest = unsafe_convert(Ptr{Cvoid}, dest)
+        psrc = unsafe_convert(Ptr{Cvoid}, src)
+        memmove(pdest, psrc, aligned_sizeof(T) * n)
+        @_gc_preserve_end tdest
+        @_gc_preserve_end tsrc
+    else
+        ccall(:jl_genericmemory_copyto, Cvoid, (Any, Ptr{Cvoid}, Any, Ptr{Cvoid}, Int), dest.mem, dest.ptr_or_offset, src.mem, src.ptr_or_offset, Int(n))
+    end
     return dest
 end