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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@18379 91177308-0d34-0410-b5e6-96231b3b80d8