tree-complex.c: fix some_nonzerop test over reals (and a bug fix)

[Laurent Thevenoux]

Hello, 

This patch improves the some_nonzerop(tree t) function from tree-complex.c file (the function is only used there). 

This function returns true if a tree as a parameter is not the constant 0 (or +0.0 only for reals when !flag_signed_zeros ). The former result is then used to determine if some simplifications are possible for complex expansions (addition, multiplication, and division). 

Unfortunately, if the tree is a real constant, the function always return true, even for +0.0 because of the explicit test on flag_signed_zeros (so if your system enables signed zeros you cannot benefit from those simplifications). To avoid this behavior and allow complex expansion simplifications, I propose the following patch, which test for the sign of the real constant 0.0 instead of checking the flag. 

This first fix reveals a bug (thanks to c-c++-common/torture/complex-sign-add.c ) in the simplification section of expand_complex_addition (also fixed in the patch). 

The patch has passed bootstrap and testing on x86_64-pc-linux-gnu . 

Best regards, 
Laurent Thévenoux 
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