ARM assembly aliases for add negative immediates using sub.

'add r2, #-1024' should just use 'sub r2, #1024' rather than erroring out.
Thumb1 aliases for adding a negative immediate to the stack pointer,
also.

rdar://11192734

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154123 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed