cmd/compile: adjust some AMD64 rewrite rules to use typed aux fields

Remove an extra int32-representable check when deciding to use an int32
constant as an immediate value.

Comment out a broken optimization that relies on MaxUint32 being
representable by a signed int32. It never triggers and when fixed, the
signedness of the auxint prevents other optimization passes from
handling it properly, thus causing segfaults in the runtime.

Remove a couple offset representable in 32-bits checks on 32-bit aux
vals.

toolstash-check clean

Change-Id: I148b53403fde523c90d692cb90e412460664b439
Reviewed-on: https://go-review.googlesource.com/c/go/+/230458


Reviewed-by: Keith Randall <khr@golang.org>