[ARM] Remove tCPS alias which just crashed

This alias caused a crash when trying to print the "cps #0" instruction in a
diagnostic for thumbv6 (which doesn't have that instruction).
	    
The comment was incorrect, this instruction is UNPREDICTABLE if no flag bits
are set, so I don't think it's worth keeping.

Differential Revision: https://reviews.llvm.org/D39191

llvm-svn: 316420