cmd/compile: SSA ".this" variable

We used to not SSA ".this" variable, because in tail-call method
wrappers it relies on updating ".this" in place in memory, and
the tail call IR node and SSA op do not have an explicit use of
".this". It is no longer the case for the new tail call
representation. Remove the restriction.

Change-Id: I4e1ce8459adbb0d5a80c64f1ece982737bd95305
Reviewed-on: https://go-review.googlesource.com/c/go/+/350751


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>