[MC] avoid create MCFillFragment when bytes is 0

[yuntaopan]

Summary

In the function emitGlobalConstantFP, we may generate a
0-byte Fill Fragment, which is followed by a Data Fragment,
that will cause the next float constant to recreate a Data
Fragment and repeat,it consumes a lot of memory when the
global variable is a floating point array.

When my global variable is a floating-point array such as [1 x [3 x [12 x [32 x [32 x float]]]]], ASM printer will consume a lot of memory, and 300MB data will consume about 30GB of memory

llvm/lib/MC/MCStreamer.cpp

 
 void MCStreamer::emitGPRel32Value(const MCExpr *Value) {
   report_fatal_error("unsupported directive in streamer");
 }
 
 /// Emit NumBytes bytes worth of the value specified by FillValue.
 /// This implements directives such as '.space'.
 void MCStreamer::emitFill(uint64_t NumBytes, uint8_t FillValue) {
+  if (NumBytes != 0)
     emitFill(*MCConstantExpr::create(NumBytes, getContext()), FillValue);
 }
 
 void llvm::MCStreamer::emitNops(int64_t NumBytes, int64_t ControlledNopLen,
                                 llvm::SMLoc, const MCSubtargetInfo& STI) {}
 
 /// The implementation in this class just redirects to emitFill.
 void MCStreamer::emitZeros(uint64_t NumBytes) { emitFill(NumBytes, 0); }