Adjust Time.argsort() for slightly faster performance.
Adjust Time.argsort() for slightly faster performance.

This came from ensuring the new Masked class worked properly with
Time, but is probably better anyway. The real change is for
`.argsort`; the ones for `.argmin` and `.argmax` are just to make the
code as similar as possible, so that it is easier to follow.

larrybradley/astropy,saimn/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,larrybradley/astropy,StuartLittlefair/astropy,astropy/astropy,pllim/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,saimn/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,mhvk/astropy,pllim/astropy,lpsinger/astropy,astropy/astropy,pllim/astropy,saimn/astropy,pllim/astropy