Revert r243347 "Add TargetTransformInfo::isZExtFree."

r243347 was intended to support a change to LSR (r243348).  That change
to LSR has since had to be reverted (r243939) because it was buggy, and
now the code added in r243347 is untested and unexercised.  Given that,
I think it is appropriate to revert r243347 for now, with the intent of
adding it back in later if I get around to checking in a fixed version
of r243348.

llvm-svn: 252948