[AArch64] Falkor supports Rounding Double Multiply Add/Subtract instructions.

Falkor only partially implements the ARMv8.1a extensions, so this patch
refactors the support for the SQRDML[A|S]H instruction into a separate
feature.

Differential Revision: https://reviews.llvm.org/D28681

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292142 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed