cmd/internal/obj/arm64: fix 32-bit BITCON test

cmd/internal/obj/arm64: fix 32-bit BITCON test

The BITCON test, isbitcon, assumes 32-bit constants are expanded
repeatedly, i.e. by copying the low 32 bits to high 32 bits,
instead of zero extending. We already do such expansion in
progedit. In con32class when classifying 32-bit constants, we
should use the expanded constant, instead of zero-extending it.

TODO: we could have better encoding for things like ANDW $-1, Rx.

Fixes #38946.

Change-Id: I37d0c95d744834419db5c897fd1f6c187595c926
Reviewed-on: https://go-review.googlesource.com/c/go/+/232984
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>