MC: Optimize getOrCreateDataFragment

... by eagerly allocating an empty fragment when adding a fragment
with a variable-size tail.

X86AsmBackend, The JCC erratum mitigation and x86-pad-for-align set a
flag for FT_Relaxable, which needs to be moved to emitInstructionBegin.
```
  if (CF->getKind() == MCFragment::FT_Relaxable)
    CF->setAllowAutoPadding(canPadInst(Inst, OS));
```

Follow-up to #148544

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index c3ecf8f..b0e52bf 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -1404,7 +1404,7 @@ MCSymbol *MCStreamer::endSection(MCSection *Section) {
   return Sym;
 }
 
-void MCStreamer::insert(MCFragment *F) {
+void MCStreamer::addFragment(MCFragment *F) {
   auto *Sec = CurFrag->getParent();
   F->setParent(Sec);
   F->setLayoutOrder(CurFrag->getLayoutOrder() + 1);
@@ -1414,7 +1414,14 @@ void MCStreamer::insert(MCFragment *F) {
 }
 
 void MCStreamer::newFragment() {
-  insert(getContext().allocFragment<MCFragment>());
+  addFragment(getContext().allocFragment<MCFragment>());
+}
+
+void MCStreamer::insert(MCFragment *F) {
+  assert(F->getKind() != MCFragment::FT_Data &&
+         "F should have a variable-size tail");
+  addFragment(F);
+  newFragment();
 }
 
 static VersionTuple