Attempt to unbreak buidlbot lld-x86_64-darwin13 after r342555.

The reason why build #25777 might have failed is because the SmallVector move
constructor is _not_ noexcept, and the stl implementation used by that buildbot
calls _VSTD::move_if_noexcept() (according to the backtrace).

OpcodeInfo has a default move constructor, and the copy constructor is deleted.
However, as far as I can see, SmallVector doesn't declare a noexcept move
constructor. So, what I believe it is happening here is that,
_VSTD::move_if_noexcept() returns an lvalue reference and not an rvalue
reference.
This eventually triggers a copy that fails to compile.

Hopefully, using a std::vector instead of SmallVector (as it was originally
suggested by Simon in the code review) should be enough to unbreak the buildbot.

llvm-svn: 342561