cmd/compile/internal/ssa: remove workarounds for #43938

The cmd/go bug this worked around should be fixed as of CL 351329.

Fixes #43938
Fixes #48550

Change-Id: Ida930e7ee33d44d89556b9b8bbc3c26bb53697b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/351529


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>