[SystemZ] Improved handling of the @llvm.ctlz intrinsic.

Since SystemZ supports counting of leading zeros with the FLOGR instruction,
isCheapToSpeculateCtlz() should return true, which it now does.

ISD::CTLZ_ZERO_UNDEF i32 is now handled the same way as ISD::CTLZ is, which
is needed since promotion to i64 is required and CTLZ_ZERO_UNDEF is only
expanded to CTLZ if it is Legal or Custom.

Review: Ulrich Weigand
https://reviews.llvm.org/D57710

llvm-svn: 353330