[llvm-exegesis] Align loop MBB in loop repetitor (#77264)

This patch sets the alignment of the loob MBB in the loop repetitor to
16 to avoid instruction fetch/predecoding bottlenecks that can come up
with unaligned code. The value of 16 was chosen based on numbers for
recent Intel microarchitectures and reccomendations from Agner Fog.

Fixes #77259.

Author: boomanaiden154

llvm/tools/llvm-exegesis/lib/SnippetRepetitor.cpp

@@ -10,6 +10,7 @@
 #include "Target.h"
 #include "llvm/ADT/Sequence.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 
 namespace llvm {
@@ -74,6 +75,11 @@ class LoopSnippetRepetitor : public SnippetRepetitor {
       auto Loop = Filler.addBasicBlock();
       auto Exit = Filler.addBasicBlock();
 
+      // Align the loop machine basic block to a target-specific boundary
+      // to promote optimal instruction fetch/predecoding conditions.
+      Loop.MBB->setAlignment(
+          Filler.MF.getSubtarget().getTargetLowering()->getPrefLoopAlignment());
+
       const unsigned LoopUnrollFactor =
           LoopBodySize <= Instructions.size()
               ? 1