ac/nir: use LLVM intrinsics for nir_op_frcp and nir_op_frsq

LLVM 11 (master) is more affected than previous versions, I assume setting the metadata doesn't really work for some reasons. I think it's much better to use the LLVM intrinsics unconditionally anyways.

VK CTS is fine on my side (tested on VEGA10).


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