On link-register machines we uses RET (sym), instead of JMP (sym),
for tail call (so the assembler knows and may rewrite it to
restore link register if necessary). Add RET to the analysis.

Fixes #17186.
Fixes #16016 on link-register machines.

Change-Id: I8690ac57dd9d49beeea76a5f291988e9a1d3afe5
Reviewed-on: https://go-review.googlesource.com/29570
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>