[NVPTX] Add a truncate DAG node to some calls.

Summary:
Previously, we were running afoul of the assertion

  EVT(CLI.Ins[i].VT) == InVals[i].getValueType() && "LowerCall emitted a value with the wrong type!"

in SelectionDAGBuilder.cpp when running the NVPTX/i8-param.ll test.
This is because our backend (for some reason) treats small return values
as i32, but it wasn't ever truncating the i32 back down to the expected
width in the DAG.

Unclear to me whether this fixes any actual bugs -- in this test, at
least, the generated code is unchanged.

Reviewers: jingyue

Subscribers: llvm-commits, tra, jholewinski

Differential Revision: http://reviews.llvm.org/D17872

llvm-svn: 265091