Fix TYPE_PRECISION use in hashable_expr_equal_p

Fix TYPE_PRECISION use in hashable_expr_equal_p

While the checks look unnecessary they probably are quick and
thus done early.  The following avoids using TYPE_PRECISION
on VECTOR_TYPEs by making the code match the comment which
talks about precision and signedness.  An alternative would
be to only retain the ERROR_MARK and TYPE_MODE checks or
use TYPE_PRECISION_RAW (but I like that least).

	* tree-ssa-scopedtables.cc (hashable_expr_equal_p):
	Use element_precision.