AsmPrinter: Compute absolute label difference directly

Create a low-overhead path for `EmitLabelDifference()` that emits a
emits an absolute number when (1) the output is an object stream and (2)
the two symbols are in the same data fragment.

This drops memory usage on Mach-O from 975 MB down to 919 MB (5.8%).
The only call is when `!doesDwarfUseRelocationsAcrossSections()` --
i.e., on Mach-O -- since otherwise an absolute offset from the start of
the section needs a relocation.  (`EmitLabelDifference()` is cheaper on
ELF anyway, since it creates 1 fewer temp symbol, and it gets called far
less often.  It's not clear to me if this is even a bottleneck there.)

(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237876 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1591,6 +1591,10 @@ void AsmPrinter::EmitInt32(int Value) const {
 /// .set if it avoids relocations.
 void AsmPrinter::EmitLabelDifference(const MCSymbol *Hi, const MCSymbol *Lo,
                                      unsigned Size) const {
+  if (!MAI->doesDwarfUseRelocationsAcrossSections())
+    if (OutStreamer->emitAbsoluteSymbolDiff(Hi, Lo, Size))
+      return;
+
   // Get the Hi-Lo expression.
   const MCExpr *Diff =
     MCBinaryExpr::CreateSub(MCSymbolRefExpr::Create(Hi, OutContext),