[llvm] Silence illegal address computation warning (PR #83244)

[Matt Arsenault via llvm-commits]

================
@@ -284,6 +284,7 @@ const RegisterBankInfo::ValueMapping *getValueMapping(unsigned BankID,
     break;
   }
 
+  assert(Idx < sizeof(ValMappings) / sizeof(ValMappings[0]));
----------------
arsenm wrote:

Can use std::size on ValMappings instead of the divide by sizeof 

https://github.com/llvm/llvm-project/pull/83244