[release-branch.go1.9] cmd/compile: make sure not to use SP as an index register
...because that's an illegal addressing mode.
I double-checked handling of this code, and 387 is the only
place where this check is missing.
Fixes #22429
Change-Id: I2284fe729ea86251c6af2f04076ddf7a5e66367c
Reviewed-on: https://go-review.googlesource.com/73551
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/88317
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>