[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

docs/Passes.rst

@@ -720,6 +720,12 @@ into the preheader block, or by sinking code to the exit blocks if it is safe.
 This pass also promotes must-aliased memory locations in the loop to live in
 registers, thus hoisting and sinking "invariant" loads and stores.
 
+Hoisting operations out of loops is a canonicalization transform. It enables
+and simplifies subsequent optimizations in the middle-end. Rematerialization
+of hoisted instructions to reduce register pressure is the responsibility of
+the back-end, which has more accurate information about register pressure and
+also handles other optimizations than LICM that increase live-ranges.
+
 This pass uses alias analysis for two purposes:
 
 #. Moving loop invariant loads and calls out of loops.  If we can determine

lib/Transforms/Scalar/LICM.cpp

@@ -12,6 +12,13 @@
 // safe.  This pass also promotes must-aliased memory locations in the loop to
 // live in registers, thus hoisting and sinking "invariant" loads and stores.
 //
+// Hoisting operations out of loops is a canonicalization transform.  It
+// enables and simplifies subsequent optimizations in the middle-end.
+// Rematerialization of hoisted instructions to reduce register pressure is the
+// responsibility of the back-end, which has more accurate information about
+// register pressure and also handles other optimizations than LICM that
+// increase live-ranges.
+//
 // This pass uses alias analysis for two purposes:
 //
 //  1. Moving loop invariant loads and calls out of loops.  If we can determine