cmd/gc: fix x=x crash
[Same as CL 102820043 except applied changes to 6g/gsubr.c also to 5g/gsubr.c and 8g/gsubr.c. The problem I had last night trying to do that was that 8g's copy of nodarg has different (but equivalent) control flow and I was pasting the new code into the wrong place.] Description from CL 102820043: 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=khr R=golang-codereviews, khr CC=golang-codereviews, iant, khr, r https://golang.org/cl/103750043
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