[llvm] [llvm-calc-occupancy] Add an AMDGPU occupancy calculator tool (PR #208727)

[Nico Weber via llvm-commits]

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+# This tool depends on AMDGPU backend internals (GCNSubtarget). Only build it
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nico wrote:

Should that be reflected in the name of the tool?

https://github.com/llvm/llvm-project/pull/208727