The table of rewrites in ssa/cse is not sized appropriately for
ssa IDs that are created during copying of selects into new blocks.

Fixes #17918

Change-Id: I65fe86c6aab5efa679aa473aadc4ee6ea882cd41
Reviewed-on: https://go-review.googlesource.com/33240Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>