[Backend] Assert that num threads is always power of 2 (NFC) (#7251) (#4899)

(cherry picked from commit 235496e)

Co-authored-by: Jeff Niu <[email protected]>

Author: whitneywhtsang

lib/Conversion/TritonGPUToLLVM/Utility.cpp

@@ -296,10 +296,9 @@ Value getThreadId(OpBuilder &rewriter, Location loc) {
     tid = rewriter.create<arith::SubIOp>(loc, tid, b.i32_val(*startId));
   }
 
-  if (llvm::isPowerOf2_32(upperBound)) {
-    // help LLVM's known bits analysis:
-    tid = b.and_(tid, b.i32_val(upperBound - 1));
-  }
+  assert(llvm::isPowerOf2_32(upperBound));
+  // help LLVM's known bits analysis:
+  tid = b.and_(tid, b.i32_val(upperBound - 1));
 
   return tid;
 }