The 'nodarg' function is used to obtain a Node*
representing a function argument or result.
It returned a brand new Node*, but that violates
the guarantee in most places in the compiler that
two Node*s refer to the same variable if and only if
they are the same Node* pointer. Reestablish that
invariant by making nodarg return a preexisting
named variable if present.

Having fixed that, avoid any copy during x=x in
componentgen, because the VARDEF we emit
before the copy marks the lhs x as dead incorrectly.

The change in walk.c avoids modifying the result
of nodarg. This was the only place in the compiler
that did so.

Fixes #8097.

LGTM=r, khr
R=golang-codereviews, r, khr
CC=golang-codereviews, iant
https://golang.org/cl/102820043