MOVWconst's AuxInt is Int32. SSA check complains if the AuxInt
does not fit in int32. Convert uint32 to int32 to make it happy.

The generated code is unchanged. MOVW only cares low 32 bits.

Passes "toolstash -cmp" std cmd for ARM.

Fixes #25993.

Change-Id: I2b6532c9c285ea6d89652505fb7c553f85a98864
Reviewed-on: https://go-review.googlesource.com/120335
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>