[vm,ia32] Avoid clobbering callee-save register in assembler test

TEST=ci
Fixes https://github.com/dart-lang/sdk/issues/62061

Change-Id: I453a7f30927562dace1d0189f29a7bd91646e3a0
Cq-Include-Trybots: luci.dart.try:vm-linux-debug-ia32-try,vm-linux-release-ia32-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/463702
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Auto-Submit: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

runtime/vm/compiler/assembler/assembler_ia32_test.cc

@@ -5092,7 +5092,8 @@ void CheckIsIdentity(Expect& expect, AssemblerTest* test) {
   }
 }
 
-static const Register kRegRegImmTempReg = EBX;
+// Use a volatile register so we don't have to save it.
+static const Register kRegRegImmTempReg = ECX;
 
 void CheckMovesInputToReturnOnly(Expect& expect,
                                  AssemblerTest* test,