[compiler] Fix printing of constant operands

The virtual register should be prefixed with a 'v' to match the printing
of virtual registers in other places.

R=mslekova@chromium.org

Bug: v8:12330
Change-Id: Ib79ace97b1c497efa3de85e1e48f5b07bb76d6cb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3358293Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78474}

src/compiler/backend/instruction.cc

@@ -178,7 +178,7 @@ std::ostream& operator<<(std::ostream& os, const InstructionOperand& op) {
       }
     }
     case InstructionOperand::CONSTANT:
-      return os << "[constant:" << ConstantOperand::cast(op).virtual_register()
+      return os << "[constant:v" << ConstantOperand::cast(op).virtual_register()
                 << "]";
     case InstructionOperand::IMMEDIATE: {
       ImmediateOperand imm = ImmediateOperand::cast(op);