[SCEV] Revise a method description to match actual behavior [NFC]

Reword the ScalarEvolution::getExitCount comment in the same terminology as used by getBackedgeTakenCount since they're equivelent for single exit loops.  Also, strengthen the comment to indicate exiting on the exact iteration specified is guaranteed.  Several transforms implicitly rely on this; and the actual implementation checks for it (via dominating latch checks).  So, spell out the guarantee in the comment.

llvm-svn: 363867