[WIP][MachineLICM] Do not rely on hasSideEffect when handling impdefs

When hasSideEffect was set to 0 in the definition of LOADgotAUTH,
the MultiSource/Benchmarks/Ptrdist/ks/ks test from llvm-test-suite
started to crash. The issue was traced down to the MachineLICM pass
placing LOADgotAUTH right after an unrelated copy to x16 like this:

    renamable $x16 = COPY renamable $x12
    renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @cost, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
    B %bb.4

Author: atrosinenko

llvm/lib/CodeGen/MachineLICM.cpp

@@ -560,18 +560,15 @@ void MachineLICMImpl::ProcessMI(MachineInstr *MI, BitVector &RUDefs,
       if (!MO.isDead())
         // Non-dead implicit def? This cannot be hoisted.
         RuledOut = true;
-      // No need to check if a dead implicit def is also defined by
-      // another instruction.
-      continue;
+    } else {
+      // FIXME: For now, avoid instructions with multiple defs, unless
+      // it's a dead implicit def.
+      if (Def)
+        RuledOut = true;
+      else
+        Def = Reg;
     }
 
-    // FIXME: For now, avoid instructions with multiple defs, unless
-    // it's a dead implicit def.
-    if (Def)
-      RuledOut = true;
-    else
-      Def = Reg;
-
     // If we have already seen another instruction that defines the same
     // register, then this is not safe.  Two defs is indicated by setting a
     // PhysRegClobbers bit.

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -1995,6 +1995,7 @@ let Predicates = [HasPAuth] in {
                Sched<[WriteI, ReadI]> {
     let Defs = [X16,X17,NZCV];
     let Size = 44;
+    let hasSideEffects = 0;
   }
 
   // Load a signed global address from a special $auth_ptr$ stub slot.