Printing: print x**-1.0 differently from 1/x

`x**-1.0` and `x**-1` should print differently.  Fixed this and added
tests.  Pretty printing seemed to work already but tests added.
Close #2860 which is related (it seems fixed but added tests).
Merge pull request #7935 from cbm755/fix_x_pow_one_float

Printing: print x**-1.0 differently from 1/x