Remove uses of the redundant ".reset(nullptr)" of unique_ptr, in favor of ".reset()"

It's also possible to just write "= nullptr", but there's some question
of whether that's as readable, so I leave it up to authors to pick which
they prefer for now. If we want to discuss standardizing on one or the
other, we can do that at some point in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213438 91177308-0d34-0410-b5e6-96231b3b80d8
ARM: correct WoA __builtin_alloca handling on O0

When performing a dynamic stack adjustment without optimisations, we would mark
SP as def and R4 as kill.  This occurred as part of the expansion of a
WIN__CHKSTK SDNode which indicated the proper handling of SP and R4.  The result
would be that we would double define SP as part of an operation, which is
obviously incorrect.

Furthermore, the VTList for the chain had an incorrect parameter type of i32
instead of Other.

Correct these to permit proper lowering of __builtin_alloca at -O0.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213442 91177308-0d34-0410-b5e6-96231b3b80d8