Revision c0e0467635ec (cmd/gc: return canonical Node* from temp)
exposed original nodes of temporaries, allowing callers to mutate
their types.

In walkcompare a temporary could be typed as ideal because of
this. Additionnally, assignment of a comparison result to
a custom boolean type was broken.

Fixes #7366.

LGTM=rsc
R=rsc, iant, khr
CC=golang-codereviews
https://golang.org/cl/66930044