isZextFree() and isTruncateFree()

[Steve King]

There was a recent commit by Sanjoy adding isZextFree().  This
reminded me that this patch:
http://reviews.llvm.org/D8304

was a related fix that I do not think got reviewed.  Can someone
please have a look?

In particular, the header comments should clarify the expected
response for corner cases like zero extending when FromTy >= ToTy and
truncating when FromType <= ToTy.  These cases do occur and violating
the undocumented assumption causes an assert.

Thanks,
-steve