[X86] Don't add implicit REP prefix to VIA PadLock xstore

[jrtc27]

Summary

Commit 8fa3e8fa1492 added an implicit REP prefix to all VIA PadLock
instructions, but GNU as doesn't add one to xstore, only all the others.
This resulted in a kernel panic regression in FreeBSD upon updating to
LLVM 11 (https://bugs.freebsd.org/259218) which includes the commit in
question. This partially reverts that commit.

[craig.topper]

LGTM

llvm/lib/Target/X86/X86InstrSystem.td

def XRSTORS64 : RI<0xC7, MRM3m, (outs), (ins opaquemem:$dst),
                    "xrstors64\t$dst",
                    [(int_x86_xrstors64 addr:$dst, EDX, EAX)]>, PS, Requires<[HasXSAVES, In64BitMode]>;
 } // Uses
 } // SchedRW
 
 //===----------------------------------------------------------------------===//
 // VIA PadLock crypto instructions
 let Defs = [RAX, RDI], Uses = [RDX, RDI], SchedRW = [WriteSystem] in
-  def XSTORE : I<0xa7, MRM_C0, (outs), (ins), "xstore", []>, TB, REP;
+  def XSTORE : I<0xa7, MRM_C0, (outs), (ins), "xstore", []>, TB;
 
 def : InstAlias<"xstorerng", (XSTORE)>;
 
 let SchedRW = [WriteSystem] in {
 let Defs = [RSI, RDI], Uses = [RBX, RDX, RSI, RDI] in {
   def XCRYPTECB : I<0xa7, MRM_C8, (outs), (ins), "xcryptecb", []>, TB, REP;
   def XCRYPTCBC : I<0xa7, MRM_D0, (outs), (ins), "xcryptcbc", []>, TB, REP;
   def XCRYPTCTR : I<0xa7, MRM_D8, (outs), (ins), "xcryptctr", []>, TB, REP;

llvm/test/MC/X86/padlock.s

 // RUN: llvm-mc -triple i386-unknown-unknown --show-encoding %s | FileCheck %s
 
+/// Check xstore does not get an implicit REP prefix but all other PadLock
+/// instructions do.
+
   xstore
 // CHECK: xstore
-// CHECK: encoding: [0xf3,0x0f,0xa7,0xc0]
+// CHECK: encoding: [0x0f,0xa7,0xc0]
   xcryptecb
 // CHECK: xcryptecb
 // CHECK: encoding: [0xf3,0x0f,0xa7,0xc8]
   xcryptcbc
 // CHECK: xcryptcbc
 // CHECK: encoding: [0xf3,0x0f,0xa7,0xd0]
   xcryptctr
 // CHECK: xcryptctr