[MSP430] Allow msp430_intrcc functions to not have interrupt attribute.

[pftbest]

Summary

Useful in case you want to have control over interrupt vector generation.
For example in Rust language we have an arrangement where all unhandled
ISR vectors gets mapped to a single default handler function. Which is
hard to implement when LLVM tries to generate vectors on its own.

[pftbest]

Add error handling when function has interrupt attribute but doesn't have correct CC.

[asl]

LGTM, thanks!

llvm/lib/Target/MSP430/MSP430AsmPrinter.cpp

void MSP430AsmPrinter::EmitInstruction(const MachineInstr *MI) {
   MCInst TmpInst;
   MCInstLowering.Lower(MI, TmpInst);
   EmitToStreamer(*OutStreamer, TmpInst);
 }
 
 void MSP430AsmPrinter::EmitInterruptVectorSection(MachineFunction &ISR) {
   MCSection *Cur = OutStreamer->getCurrentSectionOnly();
   const auto *F = &ISR.getFunction();
-  assert(F->hasFnAttribute("interrupt") &&
-         "Functions with MSP430_INTR CC should have 'interrupt' attribute");
   StringRef IVIdx = F->getFnAttribute("interrupt").getValueAsString();
   MCSection *IV = OutStreamer->getContext().getELFSection(
     "__interrupt_vector_" + IVIdx,
     ELF::SHT_PROGBITS, ELF::SHF_ALLOC | ELF::SHF_EXECINSTR);
   OutStreamer->SwitchSection(IV);
 
   const MCSymbol *FunctionSymbol = getSymbol(F);
   OutStreamer->EmitSymbolValue(FunctionSymbol, TM.getProgramPointerSize());
   OutStreamer->SwitchSection(Cur);
 }
 
 bool MSP430AsmPrinter::runOnMachineFunction(MachineFunction &MF) {
   // Emit separate section for an interrupt vector if ISR
-  if (MF.getFunction().getCallingConv() == CallingConv::MSP430_INTR)
+  if (MF.getFunction().getCallingConv() == CallingConv::MSP430_INTR &&
+      MF.getFunction().hasFnAttribute("interrupt")) {
     EmitInterruptVectorSection(MF);
+  }
 
   SetupMachineFunction(MF);
   EmitFunctionBody();
   return false;
 }
 
 // Force static initialization.
 extern "C" void LLVMInitializeMSP430AsmPrinter() {
   RegisterAsmPrinter<MSP430AsmPrinter> X(getTheMSP430Target());
 }

llvm/test/CodeGen/MSP430/interrupt.ll

; CHECK: push r4
   store volatile float %t3, float* @g
   store volatile float %t4, float* @g
   store volatile float %t5, float* @g
   store volatile float %t6, float* @g
 ; CHECK: reti
   ret void
 }
 
+; Functions without 'interrupt' attribute don't get a vector section.
+; CHECK-NOT: __interrupt_vector
+; CHECK-LABEL: NMI:
+; CHECK: reti
+define msp430_intrcc void @NMI() #1 {
+  ret void
+}
+
 attributes #0 = { noinline nounwind optnone "interrupt"="2" }
+attributes #1 = { noinline nounwind optnone }