Lower multiply-negate operation to mneg on AArch64

This patch pattern matches code such as-
neg	 w8, w8
mul	 w8, w9, w8
to
mneg	 w8, w8, w9

Review: http://reviews.llvm.org/D6754



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224706 91177308-0d34-0410-b5e6-96231b3b80d8