[LFTR] Remove unnecessary latch check; NFCI

The whole indvars pass works on loops in simplified form, so there
is always a unique latch. Convert the condition into an assertion
in needsLFTR (though we also assert this in later LFTR functions).

Additionally update the comment on getLoopTest() now that we are
dealing with multiple exits.

llvm-svn: 364713