Small optimization for complex addition, real/imag parts the same

Small optimization for complex addition, real/imag parts the same

This is just a small optimization for the case where the real and imag
parts are the same when lowering complex addition/subtraction. We only
need to do the addition once when the real and imag parts are the same (on
both sides of the operator). This gets done later on by FRE/PRE/DOM but
having it done soon allows the cabs lowering to remove the sqrt and
just change it to a multiply by a constant.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-complex.cc (expand_complex_addition): If both
	operands have the same real and imag parts, only
	add the addition once.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/complex-8.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>