[PowerPC/BlockPlacement] Allow target to provide a per-loop alignment preference

The existing code provided for specifying a global loop alignment preference.
However, the preferred loop alignment might depend on the loop itself. For
recent POWER cores, loops between 5 and 8 instructions should have 32-byte
alignment (while the others are better with 16-byte alignment) so that the
entire loop will fit in one i-cache line.

To support this, getPrefLoopAlignment has been made virtual, and can be
provided with an optional MachineLoop* so the target can inspect the loop
before answering the query. The default behavior, as before, is to return the
value set with setPrefLoopAlignment. MachineBlockPlacement now queries the
target for each loop instead of only once per function. There should be no
functional change for other targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225117 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetLowering.h

@@ -51,6 +51,7 @@ namespace llvm {
   class MachineFunction;
   class MachineInstr;
   class MachineJumpTableInfo;
+  class MachineLoop;
   class Mangler;
   class MCContext;
   class MCExpr;
@@ -929,7 +930,7 @@ public:
   }
 
   /// Return the preferred loop alignment.
-  unsigned getPrefLoopAlignment() const {
+  virtual unsigned getPrefLoopAlignment(MachineLoop *ML = nullptr) const {
     return PrefLoopAlignment;
   }
 
@@ -1336,7 +1337,8 @@ protected:
 
   /// Set the target's preferred loop alignment. Default alignment is zero, it
   /// means the target does not care about loop alignment.  The alignment is
-  /// specified in log2(bytes).
+  /// specified in log2(bytes). The target may also override
+  /// getPrefLoopAlignment to provide per-loop values.
   void setPrefLoopAlignment(unsigned Align) {
     PrefLoopAlignment = Align;
   }