CLN/PERF: delay evaluation of get_day_of_month (pandas-dev#35866)

Author: jbrockmendel

pandas/_libs/tslibs/offsets.pyx

@@ -3705,7 +3705,7 @@ cdef inline void _shift_months(const int64_t[:] dtindex,
  """See shift_months.__doc__"""
  cdef:
  Py_ssize_t i
- int months_to_roll, compare_day
+ int months_to_roll
  npy_datetimestruct dts
 
  for i in range(count):
@@ -3715,10 +3715,8 @@ cdef inline void _shift_months(const int64_t[:] dtindex,
 
  dt64_to_dtstruct(dtindex[i], &dts)
  months_to_roll = months
- compare_day = get_day_of_month(&dts, day_opt)
 
- months_to_roll = roll_convention(dts.day, months_to_roll,
- compare_day)
+ months_to_roll = _roll_qtrday(&dts, months_to_roll, 0, day_opt)
 
  dts.year = year_add_months(dts, months_to_roll)
  dts.month = month_add_months(dts, months_to_roll)