JVM_IR: rename -Xparallel-backend-threads to -Xbackend-threads

compiler/cli/cli-common/src/org/jetbrains/kotlin/cli/common/arguments/K2JVMCompilerArguments.kt

@@ -128,7 +128,8 @@ class K2JVMCompilerArguments : CommonCompilerArguments() {
     var doNotClearBindingContext: Boolean by FreezableVar(false)
 
     @Argument(
-        value = "-Xparallel-backend-threads",
+        value = "-Xbackend-threads",
+        valueDescription = "<N>",
         description = "When using the IR backend, run lowerings by file in N parallel threads.\n" +
                 "0 means use a thread per processor core.\n" +
                 "Default value is 1"

compiler/testData/cli/jvm/extraHelp.out

@@ -96,7 +96,7 @@ where advanced options include:
                                * ignore
                                * strict
                                * warn (report a warning)
-  -Xparallel-backend-threads When using the IR backend, run lowerings by file in N parallel threads.
+  -Xbackend-threads=<N>      When using the IR backend, run lowerings by file in N parallel threads.
                              0 means use a thread per processor core.
                              Default value is 1
   -Xprofile=<profilerPath:command:outputDir>