[NVPTX] Improve lowering of llvm.ctlz.

Summary:
* Disable "ctlz speculation", which inserts a branch on every ctlz(x) which
  has defined behavior on x == 0 to check whether x is, in fact zero.

* Add DAG patterns that avoid re-truncating or re-expanding the result
  of the 16- and 64-bit ctz instructions.

Reviewers: tra

Subscribers: llvm-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D28719

llvm-svn: 292299