watch for improper separation of base and exp in powsimp

e.g. sqrt(x**a) should not be split as x**a, 1/2 otherwise
it will rejoin as x**(a/2) later -- this is noted with an
inline comment.

For example, consider sqrt(exp(x)) which previously simplied
to exp(x/2). It is easy to demonstrate that these can be different:

>>> eq = sqrt(exp(x)) - exp(x/2)
>>> eq.subs(x, 3 + I).n()
-7.86610435188672 - 4.29727239280679*I
Merge pull request #7641 from smichr/powsimp

watch for improper separation of base and exp in powsimp