Fix several bugs in 'op x, imm' handling. Foremost is that we now emit

   addi r3, r3, -1
instead of
   addi r3, r3, 1

for 'sub int X, 1'.

Secondarily, this fixes several cases where we could crash given an unsigned
constant.  And fixes a couple of minor missed optimization cases, such as
xor X, ~0U -> not X

llvm-svn: 18379