[X86][MC] Emit instructions into SmallVector

Depends on D145791

Storing instruction bytes directly in a SmallVector instead of a
raw_ostream yields better encoding performance (in some applications,
the improvment is ~1% of the complete back-end time).

Reviewed By: MaskRay, Amir

Differential Revision: https://reviews.llvm.org/D145792

Author: aengelke

llvm/include/llvm/MC/MCCodeEmitter.h

@@ -37,11 +37,11 @@ class MCCodeEmitter {
   /// Lifetime management
   virtual void reset() {}
 
-  /// Emit the prefixes of given instruction on the output stream.
+  /// Append the prefixes of given instruction to the code buffer.
   ///
   /// \param Inst a single low-level machine instruction.
-  /// \param OS output stream.
-  virtual void emitPrefix(const MCInst &Inst, raw_ostream &OS,
+  /// \param CB code buffer
+  virtual void emitPrefix(const MCInst &Inst, SmallVectorImpl<char> &CB,
                           const MCSubtargetInfo &STI) const {}
   /// EncodeInstruction - Encode the given \p Inst to bytes and append to \p CB.
   virtual void encodeInstruction(const MCInst &Inst, SmallVectorImpl<char> &CB,

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -796,8 +796,7 @@ bool X86AsmBackend::padInstructionViaPrefix(MCRelaxableFragment &RF,
   const unsigned MaxPossiblePad = std::min(15 - OldSize, RemainingSize);
   const unsigned RemainingPrefixSize = [&]() -> unsigned {
     SmallString<15> Code;
-    raw_svector_ostream VecOS(Code);
-    Emitter.emitPrefix(RF.getInst(), VecOS, STI);
+    Emitter.emitPrefix(RF.getInst(), Code, STI);
     assert(Code.size() < 15 && "The number of prefixes must be less than 15.");
 
     // TODO: It turns out we need a decent amount of plumbing for the target