bugpoint should check for changes when reducing miscompile

[nlewycky]

Bugzilla Link	1527
Version	trunk
OS	All

Extended Description

As a performance hack, bugpoint should try once to see if it can reduce the set
of functions, basic blocks or instructions by comparing the code before and
after the passes.

Chances are, if a Function didn't change at all when run through the passes then
it's not related to the problem being reduced. This should happen once when
bugpoint starts looking into functions before it starts the binary search. (It
then tests whether the problem can be reproduced with the remaining functions
and if not, starts the binary cut from the whole set.)

Then once it's done the binary search on functions, it can do the same on basic
blocks before it starts doing the binary search on them. Finally, the same
applies for instructions.

You might also consider whether we can do that for the optimization passes by
comparing the code between each pass and eliminating those where the code
doesn't change, however this is less likely to work when the reduction is a
problem updating analyses.