Fix rsqrt inaccuracies. (#88707)

The current lowering has issues with large/subnormal values. This po
XLA's lowering and was verified using XLA's test suite and the
MLIR-based emitters.

This updates https://github.com/llvm/llvm-project/pull/88691 to also
update the correctness test for rsqrt(0). I checked C++ and Python, they
both agree the result should be (inf, nan). Updated the correctness test
to match this.