[libclc] [libclc] Optimize generic CLC fmin/fmax (PR #128506)

[Wenju He via cfe-commits]

wenju-he wrote:

> If the decision is the conformance test continues doing what it has been doing, it should directly map to llvm.minimumnum/maximumnum.

For now, @frasercrmck can we update this PR to use __builtin_elementwise_maximumnum/minimumnum so that OpenCL CTS can pass?

https://github.com/llvm/llvm-project/pull/128506