This is a simple patch that changes RRX and RRXS to accept all registers as operands.
According to the ARM reference manual, RRX(S) have defined encodings for lr, pc and sp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183307 91177308-0d34-0410-b5e6-96231b3b80d8