i386: Fix and improve TARGET_INDIRECT_BRANCH_REGISTER handling some more

gcc/ChangeLog:

	* config/i386/i386.md (*sibcall_pop_memory):
	Disable for TARGET_INDIRECT_BRANCH_REGISTER
	* config/i386/predicates.md (call_insn_operand): Enable when
	"satisfies_constraint_Bw (op)" is true, instead of open-coding
	constraint here.
	(sibcall_insn_operand): Ditto with "satisfies_constraint_Bs (op)"