[RISCV] Mark symbols used in inline asm for relocations as referenced

Commit 5cd8d53 taught RISCVMergeBaseOffset to handle inline asm, however
there is at least one case uncovered for integrated as.

In the example below compiler generates pcrel relocation (mcmodel=medany)
    volatile double double_val = 1.0;
    void foo() {
        asm volatile("fld f0, %0 \n\t" : : "m"(double_val) : "memory");
    }

And fails with the folliwng error
    error: could not find corresponding %pcrel_hi
          |       "fld f0, %0 \n\t"
    <inline asm>:1:2: note: instantiated into assembly here
          |         fld f0, %pcrel_lo(.Lpcrel_hi0)(a0)

After transformations MachineFunction contains inline asm instructions with
'.Lpcrel_hi0' symbol that is not defined in inline asm, but referenced.
   ... = AUIPC ...(riscv-pcrel-hi) @double_val, pre-instr-symbol <mcsymbol .Lpcrel_hi0>
   INLINEASM &"fld f0, $0 \0A\09" ... target-flags(riscv-pcrel-lo) <mcsymbol .Lpcrel_hi0>

So, when AsmParser processes 'fld', it has to create a new symbol as
'.Lpcrel_hi0' already exists but not known to be referenced in inline asm.
AsmParser avoids conflicts by renaming referenced by 'fld' symbol with
'.Lpcrel_hi00' name which does not exist. Resulting erroneous asm
    .Lpcrel_hi0:
        auipc   a0, %pcrel_hi(double_val)
        #APP
        fld     ft0, %pcrel_lo(.Lpcrel_hi00)(a0)

This change adds symbols used in memory operands to the list of referenced ones.

Author: asi-sc

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

@@ -396,6 +396,15 @@ bool RISCVAsmPrinter::PrintAsmMemoryOperand(const MachineInstr *MI,
     OS << MCO.getImm();
   else if (Offset.isGlobal() || Offset.isBlockAddress() || Offset.isMCSymbol())
     OS << *MCO.getExpr();
+
+  if (Offset.isMCSymbol())
+    MMI->getContext().registerInlineAsmLabel(Offset.getMCSymbol());
+  if (Offset.isBlockAddress()) {
+    const BlockAddress *BA = Offset.getBlockAddress();
+    MCSymbol *Sym = GetBlockAddressSymbol(BA);
+    MMI->getContext().registerInlineAsmLabel(Sym);
+  }
+
   OS << "(" << RISCVInstPrinter::getRegisterName(AddrReg.getReg()) << ")";
   return false;
 }