[disassembler] Perform comparison, not assignment

This assignment appears to have the intention of performing a comparison
and asserting that the opcode encountered was as expected, rather than
assigning it to be that particular opcode.

Change-Id: I21f3c00f414d4b191f85e9dcf4391a6dd7023495
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157105
Auto-Submit: Drew Fisher <zarvox@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Aske Simon Christensen <askesc@google.com>

runtime/vm/compiler/assembler/disassembler_x86.cc

@@ -726,7 +726,7 @@ int DisassemblerX64::ShiftInstruction(uint8_t* data) {
     Print(",%d", imm8);
     num_bytes++;
   } else {
-    ASSERT(op = 0xD2);
+    ASSERT(op == 0xD2);
     Print(",cl");
   }
   return num_bytes;