[llvm] [TTI][RISCV][SLP] Add IdentityConstantValue option to OperandValueKind for use by SLP and RISCV (PR #228630)

[Ryan Buchner via llvm-commits]

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@@ -1324,6 +1324,7 @@ class TargetTransformInfo {
     OK_AnyValue,               // Operand can have any value.
     OK_UniformValue,           // Operand is uniform (splat of a value).
     OK_UniformConstantValue,   // Operand is uniform constant.
+    OK_IdentityConstantValue,  // Operand is the identity vector (0, 1, ...)
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bababuck wrote:

I believe uniform constant is `(c, c, c, c,...)` for some constant `c`.

https://github.com/llvm/llvm-project/pull/228630