i965: Avoid applying negate to wrong MAD source.

i965: Avoid applying negate to wrong MAD source.

For some given GLSL IR like (+ (neg x) (* 1.2 x)), the try_emit_mad
function would see that one of the +'s sources was a negate expression
and set mul_negate = true without confirming that it was actually a
multiply.

Cc: 10.5 <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89315
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89095
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>