glsl: Add missing check for whether an expression is an add operation

glsl: Add missing check for whether an expression is an add operation

There is a piece of code that is trying to match expressions of the
form (mul (floor (add (abs x) 0.5) (sign x))). However the check for
the add expression wasn't checking whether it had the expected
operation. It looks like this was just an oversight because it doesn't
match the pattern for the rest of the code snippet. The existing line
to check whether add_expr!=NULL was added as part of a coverity fix in
3384179f.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91226
Cc: Matt Turner <mattst88@gmail.com>
Cc: "10.6" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>