Simplify insert/2 common case

[fhunleth]

This simplifies insertion for the case where the buffer is full and an
element is dropped. This is usually a common real world case for long
running applications.

A simple performance test showed a ~13% speed increase on an M4 MBP.

The following is a Claude-assisted diff of the bytecodes which shows an
eliminated stack frame and BIF call:

Before — ~20 instructions, 4-slot stack frame, BIF call:

  get_map_elements ... [b: x2]         # extract :b
  is_ne_exact x2, nil                  # guard b != []
  allocate 4, 3                        # ⬅︎ stack frame
  init_yregs y0
  move x2 → y1
  move x1 → y2
  move x0 → y3                         # ⬅︎ spill all three live regs
  get_map_elements ... [a: y0]         # extract :a (with deopt fallback)
  ... (deopt path through elixir_erl_pass.no_parens_remote/2) ...
  bif :tl                              # ⬅︎ tl(b) as a BIF call
  test_heap 2,2
  put_list y2, x0, x0                  # cons [item | a]
  put_map_exact ... [a, b]
  deallocate 4
  return

  After — 7 instructions, no stack frame, decomposition opcode:

  get_map_elements ... [b: x3, a: x2]  # ⬅︎ both fields in one shot
  is_nonempty_list x3                  # ⬅︎ single tag check
  test_heap 2,4
  get_tl x3 → x3                       # ⬅︎ tail-of opcode, no BIF call
  put_list x1, x2, x1                  # cons [item | a]
  put_map_exact ... [a, b]
  return