lower pow(x,1) to x (#3774)

Partially solve #3629

Currently, `genPowerWithMul()` only handles case with factor of 2 & 3,
for `y = pow(x, 1.0)`, it should be lowered to `y = x` which is much
faster than the power version.

csrc/codegen.cpp

@@ -995,32 +995,38 @@ class CudaKernelGenerator : private kir::ConstIrVisitor {
       return false;
     }
 
-    // Only **2 and **3 are considered
-    if (!(exponent == 2 || exponent == 3)) {
+    // Only **1, **2 and **3 are considered
+    if (!(exponent == 1 || exponent == 2 || exponent == 3)) {
       return false;
     }
 
     auto lhs = gen(bop->lhs());
 
     if (print_inline_) {
-      code_ << lhs << " * " << lhs;
-      if (exponent == 3) {
-        code_ << " * " << lhs;
+      for (int i = 0; i < exponent; ++i) {
+        if (i != 0) {
+          code_ << " * ";
+        }
+        code_ << lhs;
       }
     } else {
       indent() << gen(bop->out());
       if (bop->out()->isScalar()) {
-        code_ << " = " << lhs << " * " << lhs;
-        if (exponent == 3) {
-          code_ << " * " << lhs;
+        for (int i = 0; i < exponent; ++i) {
+          if (i == 0) {
+            code_ << " = " << lhs;
+          } else {
+            code_ << " * " << lhs;
+          }
         }
       } else {
-        code_ << "\n";
-        indent() << kTab << "= " << lhs << "\n";
-        indent() << kTab << "* " << lhs;
-        if (exponent == 3) {
-          code_ << "\n";
-          indent() << kTab << "* " << lhs;
+        for (int i = 0; i < exponent; ++i) {
+          if (i == 0) {
+            code_ << "\n";
+            indent() << kTab << "= " << lhs;
+          } else {
+            indent() << "\n" << kTab << "* " << lhs;
+          }
         }
       }
     }