* var-tracking.c (compute_cfa_pointer): Adjust head comment.
	(vt_initialize): Set PROLOGUE_BB unconditionally.
	Add block comment about CFA_BASE_RTX machinery.
	Reset FP_CFA_OFFSET to -1 on all invalid paths.
	Call vt_init_cfa_base only if FP_CFA_OFFSET isn't equal to -1.

From-SVN: r174146