[clang] [llvm] [SPIRV] Add OpInf support for isinf hlsl intrinsic (PR #156570)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Given the current definition of "ninf", there is no way to legally check if a value produced by an "ninf" instruction is infinity: even if the bitpattern looks like an infinity, it's actually poison.  If that's causing problems for you, we might need to revisit the definition of ninf.  It's come up a few times in the past, but we never reached consensus.

https://github.com/llvm/llvm-project/pull/156570