[codegen] Document MachineRepresentation values

If you are not using them too often, their relationship and meaning is
not totally obvious. Especially that {kTaggedSigned} means "uncompressed
Smi" cannot be inferred from the name.

R=neis@chromium.org

Bug: v8:10021
Change-Id: If684b87ba24da0bbce846d4e35a790c2ee96116b
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1967374Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65457}

src/codegen/machine-type.h

@@ -21,11 +21,11 @@ enum class MachineRepresentation : uint8_t {
   kWord16,
   kWord32,
   kWord64,
-  kTaggedSigned,
-  kTaggedPointer,
-  kTagged,
-  kCompressedPointer,
-  kCompressed,
+  kTaggedSigned,       // (uncompressed) Smi
+  kTaggedPointer,      // (uncompressed) HeapObject
+  kTagged,             // (uncompressed) Object (Smi or HeapObject)
+  kCompressedPointer,  // (compressed) HeapObject
+  kCompressed,         // (compressed) Object (Smi or HeapObject)
   // FP representations must be last, and in order of increasing size.
   kFloat32,
   kFloat64,