r600: double multiply can handle only one multiply at a time
r600: double multiply can handle only one multiply at a time

It seems clear that trying to multiply two pairs of doubles would result
in the temporary register getting overwritten by the second pair. So
make the code more explicit.

Tested-by: Glenn Kennard <feea5431f8695a3af7470d472cb16ac8e02bab01@gmail.com>
Tested-by: James Harvey <bab907bafe0121b88c465a07fe9154955b382607@gmail.com>
Cc: 17.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler