[ARM] Mark -1 as cheap in xor's for thumb1

We can always convert xor %a, -1 into MVN, even in thumb 1 where the -1
would not otherwise be considered a cheap constant. This prevents the
-1's from being pulled out into constants and potentially hoisted.

Differential Revision: https://reviews.llvm.org/D43451

llvm-svn: 325573