Remove dead predicate analysis GENERIC expr building code

Remove dead predicate analysis GENERIC expr building code

The following removes the unused def_expr, use_expr and expr APIs
from the predicate class including the unconditional build of the
GENERIC use_expr on each uninit analysis run.

	* gimple-predicate-analysis.h (predicate::m_use_expr): Remove.
	(predicate::def_expr): Likewise.
	(predicate::use_expr): Likewise.
	(predicate::expr): Likewise.
	* gimple-predicate-analysis.cc (predicate::def_expr): Remove.
	(predicate::use_expr): Likewise.
	(predicate::expr): Likewise.
	(predicate::is_use_guarded): Do not build m_use_expr.