Move cexp simplifications to match.pd

Move cexp simplifications to match.pd

This required reinstating support for captures in the result
of a simplification.  That part (genmatch.c) is by Richard B.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
2015-10-20  Richard Sandiford  <richard.sandiford@arm.com>
	    Richard Biener  <rguenther@suse.de>

	* genmatch.c (dt_simplify::gen): Skip captures that are
	part of the result.
	(parser::parse_expr): Allow captures in results too.
	* builtins.c (fold_builtin_cexp): Delete.
	(fold_builtin_1): Handle constant cexp arguments here.
	* match.pd: Fold cexp(x+yi) to exp(x) * cexpi(y).

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@229308 138bc75d-0d04-0410-961f-82ee72b054a4