[LICM] Reassociate & hoist add expressions

This patch allows LICM to reassociate and hoist following expressions:
```
loop:
  %sum = add nsw %iv, %C1
  %cmp = icmp <signed pred> %sum, C2
```
where `C1` and `C2` are loop invariants. The reassociated version looks like
```
preheader:
  %inv_sum = C2 - C1
...
loop:
  %cmp = icmp <signed pred> %iv, %inv_sum
```
In order to prove legality, we need both initial addition and the newly created subtraction
to happen without overflow.

Differential Revision: https://reviews.llvm.org/D149132
Reviewed By: skatkov