[TargetLowering][NFC] Document overloaded meaning of TypeSoftPromoteHalf

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Summary

As of D126953, TypeSoftPromoteHalf is set for bf16 if it isn't natively supported by a target. This overloads its meaning (and led to at least one bug - D157287 - due to code assuming TypeSoftPromoteHalf is only used for f16). This trivial patch documents how its meaning is overloaded.

Posting as a patch for review rather than directly committing because this seems like a good opportunity to check we're happy with overloading TypeSoftPromoteHalf's meaning.

I did grep for other TypeSoftPromoteHalf and didn't see any obvious issues. The SoftPromoteHalfResult and SoftPromoteHalfOperand code paths that gets used in this case ends up calling e.g. GetPromotionOpcode to properly handle the bf16 vs f16 case.

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llvm/include/llvm/CodeGen/TargetLowering.h

enum LegalizeTypeAction : uint8_t {
     TypePromoteInteger,  // Replace this integer with a larger one.
     TypeExpandInteger,   // Split this integer into two of half the size.
     TypeSoftenFloat,     // Convert this float to a same size integer type.
     TypeExpandFloat,     // Split this float into two of half the size.
     TypeScalarizeVector, // Replace this one-element vector with its element.
     TypeSplitVector,     // Split this vector into two of half the size.
     TypeWidenVector,     // This vector should be widened into a larger vector.
     TypePromoteFloat,    // Replace this float with a larger one.
-    TypeSoftPromoteHalf, // Soften half to i16 and use float to do arithmetic.
+    TypeSoftPromoteHalf, // Soften half/bf16 to i16 and use float to do arithmetic.
     TypeScalarizeScalableVector, // This action is explicitly left unimplemented.
                                  // While it is theoretically possible to
                                  // legalize operations on scalable types with a
                                  // loop that handles the vscale * #lanes of the
                                  // vector, this is non-trivial at SelectionDAG
                                  // level and these types are better to be
                                  // widened or promoted.
   };