[i386] Change ix86_decompose_address return type to bool.

After a recent change only a boolean value is returned.

2021-09-16  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
	* config/i386/i386-protos.h (ix86_decompose_address):
	Change return type to bool.
	* config/i386/i386.c (ix86_decompose_address): Ditto.