ART: Modernize counting-down loops

Preparation for readability-implicit-bool-conversion.

Bug: 32619234
Test: WITH_TIDY=1 mmma art
Change-Id: I7a6f9636d5f4537020f99e8cf8560f7f6b6b7ba0
diff --git a/compiler/utils/mips64/assembler_mips64.cc b/compiler/utils/mips64/assembler_mips64.cc
index 5b1c5d9..6df9562 100644
--- a/compiler/utils/mips64/assembler_mips64.cc
+++ b/compiler/utils/mips64/assembler_mips64.cc
@@ -2889,7 +2889,7 @@
     label->LinkTo(branch_id);
   }
   // Reserve space for the branch.
-  while (length--) {
+  for (; length != 0u; --length) {
     Nop();
   }
 }