ARM: Add support for the Thumb2 PLI alternate literal form.

This adds an instruction alias to make the assembler recognize the alternate literal form: pli [PC, #+/-<imm>]

See A8.8.129 in the ARM ARM (DDI 0406C.b).

Fixes <rdar://problem/14403733>.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186459 91177308-0d34-0410-b5e6-96231b3b80d8