nir/algebraic: Detect some kinds of malformed variable names

nir/algebraic: Detect some kinds of malformed variable names

I spent over an hour trying to debug a problem if a condition on a
variable not being applied.  The problem turned out to be
"a(is_not_negative" instead of "a(is_not_negative)".  This commit would
have detected that problem and failed to build.

v2: Just add $ to the end of the existing regex, and it will fail to
match a malformed string.  Suggested by Jason.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4720>