cmd/compile: delete log2, switch to log64
rewrite.go has two identical functions log2 and log64; the former has
been there for a while, while the latter was added together with
log{8,16,32} for use in typed rules.
This change deletes log2 and switches to using log64 everywhere.
Change-Id: I759b878814e4c115a5fa470274f22477738d69ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/265457
Trust: Alberto Donizetti <alb.donizetti@gmail.com>
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>