[LICM] Reassociate & hoist sub expressions

LICM could reassociate mixed variant/invariant comparison/arithmetic operations
and hoist invariant parts out of loop if it can prove that they can be computed
without overflow. Motivating example here:
```
  INV1 - VAR1 < INV2
```
can be turned into
```
  VAR > INV1 - INV2
```
if we can prove no-signed-overflow here. Then `INV1 - INV2` can be computed
out of loop, so we save one arithmetic operation in-loop.

Reviewed By: skatkov
Differential Revision: https://reviews.llvm.org/D148001

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