[XLA:GPU][IndexAnalysis] Use a flag for IsKnownEmpty instead of recomputing every time.
[XLA:GPU][IndexAnalysis] Use a flag for IsKnownEmpty instead of recomputing every time.

Right now, we would try to simplify or compose with indexing maps that have a known empty domain. That's incorrect, but checking if the domain is empty every time is expensive and can be cached.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12979 from Tixxx:tixxx/pgle_send 469b2d31ff6b0270dda28f8754462681514d0e04
