[LICM][docs] Document that LICM is also a canonicalization transform. NFC.

This documents that LICM is a canonicalization transform, which we discussed
recently in:

http://lists.llvm.org/pipermail/llvm-dev/2020-December/147184.html

but which was also discused earlier, e.g. in:

http://lists.llvm.org/pipermail/llvm-dev/2019-September/135058.html