[llvm] [Exegesis][RISCV] Add RISCV support for llvm-exegesis (PR #89047)

[Michael Maitland via llvm-commits]

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+RUN: llvm-exegesis -mode=latency --benchmark-phase=assemble-measured-code -opcode-name=LB -mtriple=riscv64-unknown-linux-gnu
+
+CHECK: Warning: Pre-assigned register prevented usage of self-aliasing strategy.
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michaelmaitland wrote:

I am a bit confused by this test. What is the pre-assigned register that prevented usage of self-aliasing? I would expect a warning that no instructions were passed.

https://github.com/llvm/llvm-project/pull/89047