[fastregalloc] Add test case for self loop (D125079).

llvm/test/CodeGen/X86/fastregalloc-selfloop.mir

@@ -0,0 +1,32 @@
+# RUN: llc -mtriple=x86_64-- -run-pass=regallocfast -o - %s | FileCheck %s
+
+...
+---
+name:            foo
+alignment:       16
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: vr128 }
+frameInfo:
+  maxAlignment:    16
+stack:
+  - { id: 0, size: 64, alignment: 16 }
+machineFunctionInfo: {}
+body:             |
+  bb.0.entry:
+    ; CHECK: renamable $xmm1 = V_SET0
+    ; CHECK-NEXT: renamable $xmm0 = V_SET0
+    ; CHECK-NEXT: renamable $xmm1 = PXORrr renamable $xmm1, renamable $xmm0
+    ; CHECK-NEXT: MOVAPSmr %stack.1, 1, $noreg, 0, $noreg, $xmm1 :: (store (s128) into %stack.1)
+    ; CHECK-NEXT: MOVAPSmr %stack.0, 1, $noreg, 0, $noreg, renamable $xmm1
+    ; CHECK-NEXT: MOVAPSmr %stack.0, 1, $noreg, 16, $noreg, killed renamable $xmm0
+
+    %0:vr128 = V_SET0
+    %1:vr128 = V_SET0
+    %0:vr128 = PXORrr %0, %1
+    MOVAPSmr %stack.0, 1, $noreg, 0, $noreg, %0
+    MOVAPSmr %stack.0, 1, $noreg, 16, $noreg, %1
+    JMP_1 %bb.0.entry
+    RET 0
+
+...