[NVPTX] Silence a GCC warning found by the buildbots

The cast to NVPTXTargetLowering was missing a 'const', but let's
just access the right pointer through the subtarget anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213793 91177308-0d34-0410-b5e6-96231b3b80d8
[NVPTX] Add some extra tests for mul.wide to test non-power-of-two source types

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213794 91177308-0d34-0410-b5e6-96231b3b80d8