[llvm] [IR] Add intrinsics to represent complex multiply and divide operations (PR #68742)

[Matt Arsenault via llvm-commits]

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@@ -18448,6 +18448,177 @@ will be on any later loop iteration.
 This intrinsic will only return 0 if the input count is also 0. A non-zero input
 count will produce a non-zero result.
 
+Complex Intrinsics
+------------------
+
+Complex numbers are currently represented, for intrinsic purposes, as vectors of
+floating-point numbers. A scalar complex type is represented using the type
+``<2 x floatty>``, with index ``0`` corresponding to the real part of the number
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arsenm wrote:

Should this be a struct instead? 

https://github.com/llvm/llvm-project/pull/68742