[NVPTX] Implement min/max in tablegen, rather than with custom DAGComine logic.

Summary:
This change also lets us use max.{s,u}16.  There's a vague warning in a
test about this maybe being less efficient, but I could not come up with
a case where the resulting SASS (sm_35 or sm_60) was different with or
without max.{s,u}16.  It's true that nvcc seems to emit only
max.{s,u}32, but even ptxas 7.0 seems to have no problem generating
efficient SASS from max.{s,u}16 (the casts up to i32 and back down to
i16 seem to be implicit and nops, happening via register aliasing).

In the absence of evidence, better to have fewer special cases, emit
more straightforward code, etc.  In particular, if a new GPU has 16-bit
min/max instructions, we want to be able to use them.

Reviewers: tra

Subscribers: jholewinski, llvm-commits

Differential Revision: https://reviews.llvm.org/D28732

llvm-svn: 292304