[MC] Optimize getCurrentSectionOnly using CurFrag and make it non-nullable

Follow-up to e48c401 ("[MC] Cache current fragment in MCStreamer").

Prerequisite: a few commits that removed nullable getCurrentSectionOnly calls.

Author: MaskRay

llvm/include/llvm/MC/MCStreamer.h

@@ -399,7 +399,9 @@ class MCStreamer {
       return SectionStack.back().first;
     return MCSectionSubPair();
   }
-  MCSection *getCurrentSectionOnly() const { return getCurrentSection().first; }
+  MCSection *getCurrentSectionOnly() const {
+    return CurFrag->getParent();
+  }
 
   /// Return the previous section that the streamer is emitting code to.
   MCSectionSubPair getPreviousSection() const {

llvm/lib/MC/MCELFStreamer.cpp

@@ -102,7 +102,7 @@ void MCELFStreamer::emitAssemblerFlag(MCAssemblerFlag Flag) {
 // needs to be aligned to at least the bundle size.
 static void setSectionAlignmentForBundling(const MCAssembler &Assembler,
                                            MCSection *Section) {
-  if (Section && Assembler.isBundlingEnabled() && Section->hasInstructions())
+  if (Assembler.isBundlingEnabled() && Section->hasInstructions())
     Section->ensureMinAlignment(Align(Assembler.getBundleAlignSize()));
 }