[llvm] [X86] Add custom operation actions for f16: FABS, FNEG, and FCOPYSIGN (PR #128877)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Just out of curiosity, what happened when i change the default (Promote) to Custom? i understand we override the default, making a custom lowering, that means we already have a implementation of custom lowering somewhere?

Yes, it needs to be set as Custom for a target's LowerOperation to be called, otherwise it will go through the generic legalization routes - take a look at SelectionDAGLegalize::LegalizeOp and how it selects the lower based on the Action.

https://github.com/llvm/llvm-project/pull/128877