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.