[PassSupport] Simplify callDefaultCtor (NFC) (llvm#137504)

We can use "constexpr if" to combine the two variants of functions.

Author: kazutakahirata

llvm/include/llvm/PassSupport.h

@@ -64,20 +64,13 @@ class Pass;
   INITIALIZE_PASS_WITH_OPTIONS_BEGIN(PassName, Arg, Name, Cfg, Analysis)       \
   INITIALIZE_PASS_END(PassName, Arg, Name, Cfg, Analysis)
 
-template <
-    class PassName,
-    std::enable_if_t<std::is_default_constructible<PassName>{}, bool> = true>
-Pass *callDefaultCtor() {
-  return new PassName();
-}
-
-template <
-    class PassName,
-    std::enable_if_t<!std::is_default_constructible<PassName>{}, bool> = true>
-Pass *callDefaultCtor() {
-  // Some codegen passes should only be testable via
-  // `llc -{start|stop}-{before|after}=<passname>`, not via `opt -<passname>`.
-  report_fatal_error("target-specific codegen-only pass");
+template <class PassName> Pass *callDefaultCtor() {
+  if constexpr (std::is_default_constructible_v<PassName>)
+    return new PassName();
+  else
+    // Some codegen passes should only be testable via
+    // `llc -{start|stop}-{before|after}=<passname>`, not via `opt -<passname>`.
+    report_fatal_error("target-specific codegen-only pass");
 }
 
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