[XLA] Make HloCSE compare computations

This shows up when you have two otherwise identical instructions that call a
computation, like a fusion or a reduce. Even if the called computations are
identical but not the same it wouldn't get CSE'd. I was a bit worried about the
compile time impact of comparing full computations, but this only happens if
everything else already compares equal. The impact on compile time of
benchmarks seems to be within the noise.

PiperOrigin-RevId: 196708782