Using --trace-wasm-decoder prints unknowns for prefix opcodes, example:

  @3      #01:Block               |  env = 0x5547c10, state = R, reason = block:start, control = #0:Start

  @4      #14:GetLocal            | i@4:GetLocal[0]
  @6      #e5:Unknown             | s@6:Unknown
  @8      #15:SetLocal            | s@8:SetLocal[1]
  @10     #14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0]
  @12     #14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1]
  @14     #cb:I8Const             | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1] i@14:I8Const
  @16     #e5:Unknown             | s@8:SetLocal[1] i@10:GetLocal[0] i@16:Unknown

Fixed to print:

  @3        #01:Block               |  env = 0x45cac10, state = R, reason = block:start, control = #0:Start

  @4        #14:GetLocal            | i@4:GetLocal[0]
  @6    #e5 #1b:I32x4Splat          | s@6:I32x4Splat
  @8        #15:SetLocal            | s@8:SetLocal[1]
  @10       #14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0]
  @12       #14:GetLocal            | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1]
  @14       #cb:I8Const             | s@8:SetLocal[1] i@10:GetLocal[0] s@12:GetLocal[1] i@14:I8Const
  @16   #e5 #1c:I32x4ExtractLane    | s@8:SetLocal[1] i@10:GetLocal[0] i@16:I32x4ExtractLane

R=ahaas@chromium.org, bbudge@chromium.org

Review-Url: https://codereview.chromium.org/2307733002
Cr-Commit-Position: refs/heads/master@{#39142}