Prevent register clobbering to fix negative zero check in SoftModulo.

This fix is not relevant for x86 architectures.
Covered by integer-division-neg2tothe32-by-neg1 JavaScript test.

Change-Id: I8ffd1280063305bc98c564f5df57c76cc5b1d7a6
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>