[llvm] [GlobalISel] Add G_STEP_VECTOR instruction (PR #115598)

[Thorsten Schütt via llvm-commits]

================
@@ -753,6 +753,25 @@ The type of the operand must be equal to or larger than the vector element
 type. If the operand is larger than the vector element type, the scalar is
 implicitly truncated to the vector element type.
 
+G_STEP_VECTOR
+^^^^^^^^^^^^^
+
+Create a scalable vector where all lanes are linear sequences starting at 0
+with a given unsigned step.
+
+The type of the operand must be equal to the vector element type. Arithmetic
+is performed modulo the bitwidth of the element. The step must be > 0.
+Otherwise the vector is zero. The canonical representation is a splat vector
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tschuett wrote:

Done.

https://github.com/llvm/llvm-project/pull/115598