[llvm] [NVPTX] Add patterns for fma.relu.{f16|bf16} (PR #114977)

[Hugh Delaney via llvm-commits]

hdelan wrote:

> Suppose the `fma` has more uses in addition to the `fmaxnum`, If this optimization kicks in it may increase the register pressure and won't be a clear win in terms of performance. I'm not sure this will be a problem, but to be conservative it may be better to implement this as a DAG combine and verify the `fma` has a single use.

I've changed the pattern matching to make sure FMA relu is only emitted if the FMA DAG has a single use.

https://github.com/llvm/llvm-project/pull/114977