[AArch64] fix an invalid-iterator-use bug.

[AArch64] fix an invalid-iterator-use bug.

Summary:
In AArch64PromoteConstant::appendAndTransferDominatedUses,
`InsertPts[NewPt]` invalidates IPI.  Therefore, `InsertPts[NewPt] =
std::move(IPI->second)` is not legal.

This was caught by running `make check` with
http://reviews.llvm.org/D7931.

Reviewers: t.p.northover, grosbach, bkramer

Reviewed By: bkramer

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D7988

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230923 91177308-0d34-0410-b5e6-96231b3b80d8