[MachineLICM] Do not rely on hasSideEffect when handling impdefs

Take clobbered registers described as implicit-def operands into
account when deciding if an instruction can be moved.

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

    bb.0:
      renamable $x16 = COPY renamable $x12
      B %bb.1

    bb.1:
      ...
      /* use $x16 */
      ...
      renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
      /* use $x20 */
      ...

like the following:

    bb.0:
      renamable $x16 = COPY renamable $x12
      renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
      B %bb.1

    bb.1:
      ...
      /* use $x16 */
      ...
      /* use $x20 */
      ...

Author: atrosinenko

llvm/lib/CodeGen/MachineLICM.cpp

@@ -559,18 +559,15 @@ 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/test/CodeGen/AArch64/mlicm-implicit-defs.mir

@@ -12,15 +12,15 @@ body: |
   ; CHECK-NEXT:   liveins: $x0
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   $x16 = COPY killed $x0
-  ; CHECK-NEXT:   $x2 = MOVi64imm 1024, implicit-def dead $x16
   ; CHECK-NEXT:   B %bb.1
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1:
   ; CHECK-NEXT:   successors: %bb.1(0x40000000), %bb.2(0x40000000)
-  ; CHECK-NEXT:   liveins: $x16, $x2
+  ; CHECK-NEXT:   liveins: $x16
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   $x1 = COPY killed $x16
-  ; CHECK-NEXT:   $x16 = LDRXroX killed $x1, $x2, 0, 0
+  ; CHECK-NEXT:   $x2 = MOVi64imm 1024, implicit-def dead $x16
+  ; CHECK-NEXT:   $x16 = LDRXroX killed $x1, killed $x2, 0, 0
   ; CHECK-NEXT:   $xzr = SUBSXri $x16, 0, 0, implicit-def $nzcv
   ; CHECK-NEXT:   Bcc 1, %bb.1, implicit $nzcv
   ; CHECK-NEXT:   B %bb.2