Remove requiresDiffExpressionRelocations target hook

Turns out this isn't needed, each fragment records whether it's relaxable

Author: jmorse

llvm/include/llvm/MC/MCAsmBackend.h

@@ -135,10 +135,6 @@ class MCAsmBackend {
                           uint64_t Value, bool IsResolved,
                           const MCSubtargetInfo *STI) const = 0;
 
-  /// Check whether the given target requires emitting differences of two
-  /// symbols as a set of relocations.
-  virtual bool requiresDiffExpressionRelocations() const { return false; }
-
   /// @}
 
   /// \name Target Relaxation Interfaces

llvm/lib/MC/MCObjectStreamer.cpp

@@ -469,14 +469,12 @@ void MCObjectStreamer::emitDwarfAdvanceLineAddr(int64_t LineDelta,
   // fragment, then the address-offset is already a fixed constant and is not
   // relaxable. Emit the advance-line-addr data immediately to save time and
   // memory.
-  if (!Assembler->getBackend().requiresDiffExpressionRelocations()) {
-    if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
-      SmallString<16> Tmp;
-      MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
-                              LineDelta, *OptAddrDelta, Tmp);
-      emitBytes(Tmp);
-      return;
-    }
+  if (auto OptAddrDelta = absoluteSymbolDiff(Label, LastLabel)) {
+    SmallString<16> Tmp;
+    MCDwarfLineAddr::encode(getContext(), Assembler->getDWARFLinetableParams(),
+                            LineDelta, *OptAddrDelta, Tmp);
+    emitBytes(Tmp);
+    return;
   }
 
   const MCExpr *AddrDelta = buildSymbolDiff(*this, Label, LastLabel, SMLoc());

llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.h

@@ -35,20 +35,6 @@ class RISCVAsmBackend : public MCAsmBackend {
 
   void setForceRelocs() { ForceRelocs = true; }
 
-  // Returns true if relocations will be forced for shouldForceRelocation by
-  // default. This will be true if relaxation is enabled or had previously
-  // been enabled.
-  bool willForceRelocations() const {
-    return ForceRelocs || STI.getFeatureBits()[RISCV::FeatureRelax];
-  }
-
-  // Generate diff expression relocations if the relax feature is enabled or had
-  // previously been enabled, otherwise it is safe for the assembler to
-  // calculate these internally.
-  bool requiresDiffExpressionRelocations() const override {
-    return willForceRelocations();
-  }
-
   // Return Size with extra Nop Bytes for alignment directive in code section.
   bool shouldInsertExtraNopBytesForCodeAlign(const MCAlignFragment &AF,
                                              unsigned &Size) override;