simplifier: move if_expr related methods into separate file

These have tight coupling, and there is a sufficient number of them.

Author: Daniel Kroening

src/util/Makefile

@@ -69,6 +69,7 @@ SRC = allocate_objects.cpp \
       simplify_expr_array.cpp \
       simplify_expr_boolean.cpp \
       simplify_expr_floatbv.cpp \
+      simplify_expr_if.cpp \
       simplify_expr_int.cpp \
       simplify_expr_pointer.cpp \
       simplify_expr_struct.cpp \