Thanks to sabre for pointing out that we were incorrectly codegen'ing

Thanks to sabre for pointing out that we were incorrectly codegen'ing
int test(int x) { return 32768 - x; }

Fixed by teaching the function that checks a constant's validity to be used
as an immediate argument about subtract-from instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@17476 91177308-0d34-0410-b5e6-96231b3b80d8