cmd/compile: use standard dom tree in nilcheckelim

cmd/compile: use standard dom tree in nilcheckelim

No need to build a bespoke dom tree here when we might
have one cached already.  The allocations for the dom tree
were also more expensive than they needed to be.

Fixes #12021

Change-Id: I6a967880aee03660ad6fc293f8fc783779cae11d
Reviewed-on: https://go-review.googlesource.com/30671
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>