Fix request shift queue not enqueuing when empty

The queue was enabling shifting of the registers whenever deq.ready
was 1, even when the queue was empty.  This caused `wen` to disable
writing enq.bits to any of the entries in the queue.  Fixed by setting
`shift` to 0 when queue is empty.

src/test/scala/coalescing/CoalescingUnitTest.scala

@@ -87,7 +87,7 @@ class CoalShiftQueueTest extends AnyFlatSpec with ChiselScalatestTester {
       c.io.invalidate.poke(0.U)
 
       // prepare
-      c.io.deq.ready.poke(false.B)
+      c.io.deq.ready.poke(true.B)
       c.io.enq.ready.expect(true.B)
       c.io.enq.valid.poke(true.B)
       c.io.enq.bits.poke(0x12.U)
@@ -113,6 +113,45 @@ class CoalShiftQueueTest extends AnyFlatSpec with ChiselScalatestTester {
     }
   }
 
+  it should "work when enqueing and dequeueing simultaneously to a full queue" in {
+    test(new CoalShiftQueue(UInt(8.W), 1)) { c =>
+      c.io.invalidate.poke(0.U)
+
+      // prepare
+      c.io.deq.ready.poke(true.B)
+      c.io.enq.ready.expect(true.B)
+      c.io.enq.valid.poke(true.B)
+      c.io.enq.bits.poke(0x12.U)
+      c.clock.step()
+      // enqueue and dequeue simultaneously
+      c.io.deq.ready.poke(true.B)
+      c.io.enq.ready.expect(true.B)
+      c.io.enq.valid.poke(true.B)
+      c.io.enq.bits.poke(0x34.U)
+      c.io.deq.valid.expect(true.B)
+      c.io.deq.bits.expect(0x12.U)
+      c.clock.step()
+      // enqueue and dequeue simultaneously once more
+      c.io.deq.ready.poke(true.B)
+      c.io.enq.ready.expect(true.B)
+      c.io.enq.valid.poke(true.B)
+      c.io.enq.bits.poke(0x56.U)
+      c.io.deq.valid.expect(true.B)
+      c.io.deq.bits.expect(0x34.U)
+      c.clock.step()
+      // dequeueing back-to-back should work without any holes in the middle
+      c.io.deq.ready.poke(true.B)
+      c.io.enq.valid.poke(false.B)
+      c.io.deq.valid.expect(true.B)
+      c.io.deq.bits.expect(0x56.U)
+      c.clock.step()
+      // make sure is empty
+      c.io.deq.ready.poke(true.B)
+      c.io.enq.valid.poke(false.B)
+      c.io.deq.valid.expect(false.B)
+    }
+  }
+
   it should "invalidate head being dequeued" in {
     test(new CoalShiftQueue(UInt(8.W), 4)) { c =>
       c.io.invalidate.poke(0.U)