[LICM] Only recompute LCSSA when we actually promoted something.

We want to recompute LCSSA only when we actually promoted a value.
This means we only need to look at changes made by promotion when
deciding whether to recompute it or not, not at regular sinking/hoisting.

(This was what the code was documented as doing, just not what it did)

Hopefully NFC.

llvm-svn: 290726