[AArch64]: add 'a' inline asm operand modifier.

[manojgupta]

Summary

This is used in the Linux kernel, and effectively just means "print an
address". This brings back r193593.

[rengolin]

Silly inline comment, but otherwise, LGTM. Thanks!

lib/Target/AArch64/AArch64AsmPrinter.cpp

395

why not:

if (ExtraCode && ExtraCode[0] && ExtraCode[0] != 'a')
  return true;

lib/Target/AArch64/AArch64AsmPrinter.cpp

     switch (ExtraCode[0]) {
     default:
       return true; // Unknown modifier.
+    case 'a':      // Print 'a' modifier
+      PrintAsmMemoryOperand(MI, OpNum, AsmVariant, ExtraCode, O);
+      return false;
     case 'w':      // Print W register
     case 'x':      // Print X register
       if (MO.isReg())

                                               unsigned AsmVariant,
                                               const char *ExtraCode,
                                               raw_ostream &O) {
-  if (ExtraCode && ExtraCode[0])
+  if (ExtraCode && ExtraCode[0] && ExtraCode[0] != 'a')
     return true; // Unknown modifier.
 
   const MachineOperand &MO = MI->getOperand(OpNum);

test/CodeGen/AArch64/arm64-inline-asm.ll

   tail call void asm sideeffect "sqxtn h0, ${0:s}\0A", "w"(i32 %a)
   ret void
 }
+
+define void @test_inline_modifier_a(i8* %ptr) nounwind {
+  ; CHECK-LABEL: test_inline_modifier_a:
+  tail call void asm sideeffect "prfm pldl1keep, ${0:a}\0A", "r"(i8* %ptr)
+  ; CHECK: prfm pldl1keep, [x0]
+  ret void
+}