Take fields into account checking for self-assign

Take fields into account checking for self-assign

The code in `fgMorphCopyBlock` that removes self-assigns checks if the LHS
and RHS refer to the same `lclVar`; update it to also check if the LHS and
RHS refer to the same field(s) of that `lclVar`, since otherwise copies
from one field to another of a struct can get lost.

Fixes dotnet/coreclr#10481.

Commit migrated from https://github.com/dotnet/coreclr/commit/c3610d32da61d995191d77b91735b462b46a516a