[AArch64] Strengthen the code for the prologue insertion.
The spilled registers are pristine and thus, correctly handled by
the register scavenger and so on, but the liveness information is
strictly speaking wrong at this point.
Fix that.

llvm-svn: 234664
diff --git a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
index 01716c3..bd2af16 100644
--- a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -787,6 +787,8 @@
     if (StrOpc == AArch64::STPDpre || StrOpc == AArch64::STPXpre)
       MIB.addReg(AArch64::SP, RegState::Define);
 
+    MBB.addLiveIn(Reg1);
+    MBB.addLiveIn(Reg2);
     MIB.addReg(Reg2, getPrologueDeath(MF, Reg2))
         .addReg(Reg1, getPrologueDeath(MF, Reg1))
         .addReg(AArch64::SP)