cmd/compile: make rulegen magic variable prediction more precise

The sheer length of the generated rules files makes my
editor and git client unhappy.
This change is a small step towards shortening them.

We recognize a few magic variables during rulegen: b, config, fe, typ.
Of these, only b appears prone to false positives.
By tightening the heuristic and fixing one case in MIPS.rules,
we can make the heuristic enough that it has no failures.
That allows us to remove the hedge assignments to _,
removing 3000 pointless lines of code.

Change-Id: I080cde5db28c8277cb3fd9ddcd829306c9a27785
Reviewed-on: https://go-review.googlesource.com/c/go/+/166979


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>