[NVPTX] Make the legalizer expand shufflevector of <2 x half>

There's no direct instruction for this, but it's trivially implemented
with two movs. Without this the code generator just dies when
encountering a shufflevector.

Differential Revision: https://reviews.llvm.org/D46116

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330948 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed