Fix testing of the VEX.L (128/256-bit) flag in the x64 disassembler

The current code for testing the VEX.L flag, indicating whether
128-bit or 256-bit registers are being accessed, was erroneous
and always returned true (i.e. indicated 128-bit registers).

This patch fixes this behaviour and checks the flag correctly.

Ref: https://github.com/nodejs/node/issues/6151

BUG=

Review URL: https://codereview.chromium.org/1875323002

Cr-Commit-Position: refs/heads/master@{#35506}

AUTHORS

@@ -40,6 +40,7 @@ Alexis Campailla <alexis@janeasystems.com>
 Andreas Anyuru <andreas.anyuru@gmail.com>
 Andrew Paprocki <andrew@ishiboo.com>
 Andrei Kashcha <anvaka@gmail.com>
+Anna Henningsen <addaleax@gmail.com>
 Bangfu Tao <bangfu.tao@samsung.com>
 Ben Noordhuis <info@bnoordhuis.nl>
 Benjamin Tan <demoneaux@gmail.com>

src/ia32/disasm-ia32.cc

@@ -282,7 +282,7 @@ class DisassemblerIA32 {
   bool vex_128() {
     DCHECK(vex_byte0_ == 0xc4 || vex_byte0_ == 0xc5);
     byte checked = vex_byte0_ == 0xc4 ? vex_byte2_ : vex_byte1_;
-    return (checked & 4) != 1;
+    return (checked & 4) == 0;
   }
 
   bool vex_none() {

src/x64/disasm-x64.cc

@@ -360,7 +360,7 @@ class DisassemblerX64 {
   bool vex_128() {
     DCHECK(vex_byte0_ == VEX3_PREFIX || vex_byte0_ == VEX2_PREFIX);
     byte checked = vex_byte0_ == VEX3_PREFIX ? vex_byte2_ : vex_byte1_;
-    return (checked & 4) != 1;
+    return (checked & 4) == 0;
   }
 
   bool vex_none() {