Improve support for IMAGPART_EXPR and REALPART_EXPR in ranger.

Improve support for IMAGPART_EXPR and REALPART_EXPR in ranger.

Currently we adjust statements containing an IMAGPART_EXPR if the
defining statement was one of a few built-ins known to return boolean
types.  We can also adjust statements for both IMAGPART_EXPR and
REALPART_EXPR where the defining statement is a constant.

This patch adds such support, and cleans up the code a bit.

Tested on x86-64 Linux.

gcc/ChangeLog:

	* gimple-range-fold.cc (adjust_imagpart_expr): Move from
	gimple_range_adjustment.  Add support for constants.
	(adjust_realpart_expr): New.
	(gimple_range_adjustment): Move IMAGPART_EXPR code to
	adjust_imagpart_expr.
	* range-op.cc (integral_table::integral_table): Add entry for
	REALPART_CST.