IR: an option to automatically select the number of lowering threads

compiler/testData/cli/jvm/extraHelp.out

@@ -90,6 +90,9 @@ where advanced options include:
   -Xno-receiver-assertions   Don't generate not-null assertion for extension receiver arguments of platform types
   -Xno-reset-jar-timestamps  Do not reset jar entry timestamps to a fixed date
   -Xno-unified-null-checks   Use pre-1.4 exception types in null checks instead of java.lang.NPE. See KT-22275 for more details
+  -Xparallel-backend-threads 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>
                              Debug option: Run compiler with async profiler, save snapshots to outputDir, command is passed to async-profiler on start
                              You'll have to provide async-profiler.jar on classpath to use this
@@ -122,9 +125,6 @@ where advanced options include:
                              Suppress deprecation warning about deprecated JVM target versions
   -Xsuppress-missing-builtins-error
                              Suppress the "cannot access built-in declaration" error (useful with -no-stdlib)
-  -Xir-threads-for-file-lowerings
-                             When using the IR backend, run lowerings by file in N parallel threads.
-                             Default value is 1
   -Xuse-ir                   Use the IR backend
   -Xuse-javac                Use javac for Java source and class files analysis
   -Xuse-old-backend          Use the old JVM backend
@@ -193,3 +193,4 @@ where advanced options include:
 
 Advanced options are non-standard and may be changed or removed without any notice.
 OK
+