cmd/compile: set n.Name.Defn for inlined parameters

Normally, when variables are declared and initialized using ":=", we
set the variable's n.Name.Defn to point to the initialization
assignment node (i.e., OAS or OAS2). Further, some frontend
optimizations look for variables that are initialized but never
reassigned.

However, when inl.go inlines calls, it was declaring the inlined
variables, and then separately assigning to them. This CL changes
inl.go tweaks the AST to fit the combined declaration+initialization
pattern.

This isn't terribly useful by itself, but it allows further followup
optimizations.

Updates #41474.

Change-Id: I62a9752c60414305679e0ed15a6563baa0224efa
Reviewed-on: https://go-review.googlesource.com/c/go/+/256457


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