[AArch64]Select wide immediate offset into [Base+XReg] addressing mode
e.g Currently we'll generate following instructions if the immediate is too wide:
    MOV  X0, WideImmediate
    ADD  X1, BaseReg, X0
    LDR  X2, [X1, 0]

    Using [Base+XReg] addressing mode can save one ADD as following:
    MOV  X0, WideImmediate
    LDR  X2, [BaseReg, X0]

    Differential Revision: http://reviews.llvm.org/D5477


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219665 91177308-0d34-0410-b5e6-96231b3b80d8