reflect: undo register count increments on register assignment failure

Currently when register assignment fails we roll back all the abiParts
that were generated in the process. However, the total number of
registers also increases, but does not get rolled back. The result is
a very incorrect register assignment.

For #40724.
For #44816.

Change-Id: I1934ea5f95f7608ff2067166255099dbc9135e8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/306109


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>