[clang] [llvm] [RFC][CodeGen] Add generic target feature checks for intrinsics (PR #201470)

[Jay Foad via cfe-commits]

jayfoad wrote:

I agree with @ssahasra, I would prefer to reduce the scope of this PR by dropping all the `$custom` stuff. It can always be added later if it has merit.

https://github.com/llvm/llvm-project/pull/201470