patch 8.1.2077: the ops.c file is too big

Problem:    The ops.c file is too big.
Solution:   Move code for dealing with registers to a new file. (Yegappan
            Lakshmanan, closes #4982)
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index 6b549df..0f3cd4e 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -85,6 +85,7 @@
 	profiler.c						\
 	quickfix.c						\
 	regexp.c						\
+	register.c						\
 	scriptfile.c						\
 	screen.c						\
 	search.c						\