[BACKEND] Fix the bug in software loop pipeline (#2737)

### Issue
The software loop pipeline used the wrong predication for each stage in
pre-epilogue.
### Fix
1. Using the array to keep reference to each predication value when
created.
2. Refer the correct predication value for the ops in each stage which
is just like the ops iterated before pipelining.

No change to the stage scheduling sequence. There should no impacting on
the performance.

lib/Dialect/TritonGPU/Transforms/Pipeliner/PipelineExpander.cpp

@@ -223,8 +223,8 @@ void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
   }
   auto yield = cast<scf::YieldOp>(forOp.getBody()->getTerminator());
   Location loc = forOp.getLoc();
+  SmallVector<Value, 5> predicates(maxStage);
   for (int64_t i = 0; i < maxStage; i++) {
-    Value predicate;
     if (dynamicLoop) {
       Type t = ub.getType();
       // pred = ub > lb + (i * step)
@@ -234,8 +234,8 @@ void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
               loc, step,
               rewriter.create<arith::ConstantOp>(
                   loc, rewriter.getIntegerAttr(t, i))));
-      predicate = rewriter.create<arith::CmpIOp>(loc, arith::CmpIPredicate::slt,
-                                                 iv, ub);
+      predicates[i] = rewriter.create<arith::CmpIOp>(
+          loc, arith::CmpIPredicate::slt, iv, ub);
     }
 
     // special handling for induction variable as the increment is implicit.
@@ -259,8 +259,9 @@ void LoopPipelinerInternal::emitPrologue(RewriterBase &rewriter) {
               newOperand->set(replacement);
             }
           });
-      if (predicate) {
-        newOp = predicateFn(rewriter, newOp, predicate);
+      int predicateIdx = i - stages[op];
+      if (predicates[predicateIdx]) {
+        newOp = predicateFn(rewriter, newOp, predicates[predicateIdx]);
         assert(newOp && "failed to predicate op.");
       }
       if (annotateFn)