[x86] add test/code comment for chain value use (PR46195); NFC
I'm pretty new to how testing is usually done in LLVM - is it common to do exact matching against generated assembly? I was worried that unrelated changes affecting the generated code would need to update this test.
That's a valid concern. Exact matching does tend to increase test churn if someone wants to make an unrelated change for example to code scheduling or register allocation.
For x86 codegen, we are generally willing to do exact matching because it has proven useful even with the potential upkeep cost.
See PR22897 for background: