Correct lowering of memmove in NVPTX

This fixes https://llvm.org/bugs/show_bug.cgi?id=24056

Also a bit of refactoring along the way.

Differential Revision: http://reviews.llvm.org/D11220


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242413 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed