[llvm] [llvm-exegesis] Add CLI Option to set Fixed RNG seed (PR #170013)

[David Green via llvm-commits]

================
@@ -187,8 +195,13 @@ generateUnconstrainedCodeTemplates(const InstructionTemplate &Variant,
 }
 
 std::mt19937 &randomGenerator() {
+  unsigned RandomSeed = RandomGeneratorSeed;
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davemgreen wrote:

```
RandomSeed = RandomGeneratorSeed.getNumOccurrences() ? RandomGeneratorSeed : RandomDevice();
```
This can allow a seed of 0 too.

https://github.com/llvm/llvm-project/pull/170013