Remove undefined behaviour from builtins-20.c

Remove undefined behaviour from builtins-20.c

builtins-20.c had:

      if (cos((y*=2, -fabs(tan(x/-y)))) != cos((y*=2,tan(x/y))))
        link_error ();

which is undefined behaviour.  The test expected that y had the same
value in x/y and x/-y, but gimplification actually implements the
"obvious" interpretation, multiplying y by 2, using it for one cos call,
then multiplying it by 2 again and using it for the other cos call.

The file has other (valid) tests that side-effects don't block
optimisation, such as:

      if (cosf((y*=3, -x)) != cosf((y*=3,x)))
        link_error ();

so this patch simply removes this instance.

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

gcc/testsuite/
	* gcc.dg/builtins-20.c: Remove undefined behavior.

From-SVN: r228963