X86AsmBackend: Remove some computeFragmentSize calls

The function was brittle as it performed only one iteration, which might
not converge. The reverted be5a845
mentioned that !NDEBUG and NDEBUG builds evaluated fragment offsets at
different times.

    * X86AsmBackend::finishLayout performed only one iteration, which might
      not converge. In addition, the removed `#ifndef NDEBUG` code (disabled
      by default) in X86AsmBackend::finishLayout was problematic, as !NDEBUG
      and NDEBUG builds evaluated fragment offsets at different times before
      this patch.

Author: MaskRay

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

@@ -916,9 +916,6 @@ void X86AsmBackend::finishLayout(MCAssembler const &Asm) const {
         continue;
       }
 
-#ifndef NDEBUG
-      const uint64_t OrigOffset = Asm.getFragmentOffset(F);
-#endif
       const uint64_t OrigSize = Asm.computeFragmentSize(F);
 
       // To keep the effects local, prefer to relax instructions closest to
@@ -949,14 +946,6 @@ void X86AsmBackend::finishLayout(MCAssembler const &Asm) const {
       if (F.getKind() == MCFragment::FT_BoundaryAlign)
         cast<MCBoundaryAlignFragment>(F).setSize(RemainingSize);
 
-#ifndef NDEBUG
-      const uint64_t FinalOffset = Asm.getFragmentOffset(F);
-      const uint64_t FinalSize = Asm.computeFragmentSize(F);
-      assert(OrigOffset + OrigSize == FinalOffset + FinalSize &&
-             "can't move start of next fragment!");
-      assert(FinalSize == RemainingSize && "inconsistent size computation?");
-#endif
-
       // If we're looking at a boundary align, make sure we don't try to pad
       // its target instructions for some following directive.  Doing so would
       // break the alignment of the current boundary align.
@@ -971,10 +960,8 @@ void X86AsmBackend::finishLayout(MCAssembler const &Asm) const {
   }
 
   // The layout is done. Mark every fragment as valid.
-  for (MCSection &Section : Asm) {
+  for (MCSection &Section : Asm)
     Asm.getFragmentOffset(*Section.curFragList()->Tail);
-    Asm.computeFragmentSize(*Section.curFragList()->Tail);
-  }
 }
 
 unsigned X86AsmBackend::getMaximumNopSize(const MCSubtargetInfo &STI) const {