[llvm] [GlobalISel] Reorder code in CombinerHelper::buildUDivUsingMul. NFC (PR #99565)

[Matt Arsenault via llvm-commits]

================
@@ -5213,6 +5209,34 @@ MachineInstr *CombinerHelper::buildUDivUsingMul(MachineInstr &MI) {
     return true;
   };
 
+  if (MI.getFlag(MachineInstr::MIFlag::IsExact)) {
+    // Collect all magic values from the build vector.
+    bool Matched = matchUnaryPredicate(MRI, RHS, BuildExactUDIVPattern);
+    (void)Matched;
+    assert(Matched && "Expected unary predicate match to succeed");
----------------
arsenm wrote:

```suggestion
    if (!matchUnaryPredicate(MRI, RHS, BuildExactUDIVPattern))
      llvm_unreachable("Expected unary predicate match to succeed");
```

https://github.com/llvm/llvm-project/pull/99565