cmd/compile/internal/ssa: remove linkedBlocks and its uses
The use of predFn/succFn is not needed since CL 22401.
Change-Id: Icc39190bb7b0e85541c75da2d564093d551751d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/659555
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>