[dev.regabi] cmd/compile: do not rely on CallExpr.Rargs for detect already walked calls

Currently, there's an awkward issue with walk pass. When walking the AST
tree, the compiler generate code for runtime functions (using mkcall* variants),
add/modify the AST tree and walk new generated tree again. This causes the
double walking on some CallExpr, which is relying on checking Rargs to prevent
that. But checking Rargs has its own issue as well.

For functions that does not have arguments, this check is failed, and we
still double walk the CallExpr node.

This CL change the way that compiler detects double walking, by using
separated field instead of relying on Rargs. In perfect world, we should make
the compiler walks the AST tree just once, but it's not safe to do that at
this moment.

Passes toolstash -cmp.

Change-Id: Ifdd1e0f98940ddb1f574af2da2ac7f005b5fcadd
Reviewed-on: https://go-review.googlesource.com/c/go/+/283672


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>