[x64] Add disassembler support for 'pause'

The 'pause' instruction is used for implementing retpolines. It is
currently being printed as 'nop', which is incorrect.

R=titzer@chromium.org

Change-Id: I134b6dae332103fd7f9b3c4e5520f0d5db06ba74
Reviewed-on: https://chromium-review.googlesource.com/1051789Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53109}

src/x64/disasm-x64.cc

@@ -2567,7 +2567,9 @@ int DisassemblerX64::InstructionDecode(v8::internal::Vector<char> out_buffer,
       case 0x96:
       case 0x97: {
         int reg = (*data & 0x7) | (rex_b() ? 8 : 0);
-        if (reg == 0) {
+        if (group_1_prefix_ == 0xF3 && *data == 0x90) {
+          AppendToBuffer("pause");
+        } else if (reg == 0) {
           AppendToBuffer("nop");  // Common name for xchg rax,rax.
         } else {
           AppendToBuffer("xchg%c rax,%s",

test/cctest/test-disasm-x64.cc

@@ -955,6 +955,7 @@ TEST(DisasmX64) {
     __ Nop(i);
   }
 
+  __ pause();
   __ ret(0);
 
   CodeDesc desc;