IC mangling: Use '_' instead of 'x' as a placeholder before hashing

compiler/testData/writeSignature/inlineClasses/genericMethodInsideClass.kt

@@ -22,6 +22,6 @@ inline class SomeClass(val v: Int) {
 // jvm signature: (ILjava/lang/Object;)Ljava/lang/Object;
 // generic signature: <K:Ljava/lang/Object;>(ITK;)TK;
 
-// method: SomeClass$Companion::comp-aew-wRw
+// method: SomeClass$Companion::comp-TwaN_yk
 // jvm signature: (ILjava/lang/Object;)Ljava/lang/Object;
 // generic signature: <T:Ljava/lang/Object;>(ITT;)TT;

compiler/testData/writeSignature/inlineClasses/simpleSignatureWithInlineClassTypesAsPrimitive.kt

@@ -21,6 +21,6 @@ object Test {
 // jvm signature: (I)V
 // generic signature: null
 
-// method: Test::asAll-wqC9CcI
+// method: Test::asAll-ZcpZUGQ
 // jvm signature: (ILjava/lang/Object;II)I
 // generic signature: null