Add/Sub/Mul are safe to promote here as well. Incrementing a single-bit

Add/Sub/Mul are safe to promote here as well.  Incrementing a single-bit
bitfield now gives this code:

_plus:
        lwz r2, 0(r3)
        rlwimi r2, r2, 0, 1, 31
        xoris r2, r2, 32768
        stw r2, 0(r3)
        blr

instead of this:

_plus:
        lwz r2, 0(r3)
        srwi r4, r2, 31
        slwi r4, r4, 31
        addis r4, r4, -32768
        rlwimi r2, r4, 0, 0, 0
        stw r2, 0(r3)
        blr

this can obviously still be improved.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28275 91177308-0d34-0410-b5e6-96231b3b80d8