cmd/compile: make pre-elimination of rulegen bounds checks more precise

In cases in which we had a named value whose args were all _,
like this rule from ARM.rules:

(MOVBUreg x:(MOVBUload _ _)) -> (MOVWreg x)

We previously inserted

_ = x.Args[1]

even though it is unnecessary.
This change eliminates this pointless bounds check.
And in other cases, we now check bounds just as far as strictly necessary.

No significant movement on any compiler metrics.
Just nicer (and less) code.

Passes toolstash-check -all.

Change-Id: I075dfe9f926cc561cdc705e9ddaab563164bed3a
Reviewed-on: https://go-review.googlesource.com/c/go/+/221781


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>