[ARM] Do not generate invalid encoding for stack adjust, even if this is just

[ARM] Do not generate invalid encoding for stack adjust, even if this is just
temporary.

Because of that:
1. The machine verifier was complaining on such code.
2. The generate code worked just because the thumb reduction size pass fixed the
opcode.

rdar://problem/20749824

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236247 91177308-0d34-0410-b5e6-96231b3b80d8