RegisterScavenger: Take a reference as enterBasicBlock() argument.

Make it obvious that the argument cannot be nullptr.
Remove an unnecessary nullptr check in initRegState.

llvm-svn: 265511
diff --git a/llvm/lib/Target/PowerPC/PPCFrameLowering.cpp b/llvm/lib/Target/PowerPC/PPCFrameLowering.cpp
index 1d59228..aadf64d 100644
--- a/llvm/lib/Target/PowerPC/PPCFrameLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCFrameLowering.cpp
@@ -596,7 +596,7 @@
       (!UseAtEnd && (&MBB->getParent()->front() == MBB)))
     return true;
 
-  RS.enterBasicBlock(MBB);
+  RS.enterBasicBlock(*MBB);
 
   if (UseAtEnd && !MBB->empty()) {
     // The scratch register will be used at the end of the block, so must