Fix for issue 5954 - Assert 'fieldSeq != FieldSeqStore::NotAField()'

Fix for issue 5954 - Assert 'fieldSeq != FieldSeqStore::NotAField()'

Added new method GenTree:OperIsLocalField()
Fixes DefinesLocalAddress to properly update pIsEntire when we have a LocalField node type
Fixed fgValueNumberBlockAssignment to handle a GT_COPYBLK or GT_COPYOBJ operation
  where we have an incomplete field sequence for the Dst/LHS node.
  We already were handling an incomplete field sequence for the Src/RHS node.
  Update the dump to properly print "new uniq" whenever we issue a conservative value number.

Commit migrated from https://github.com/dotnet/coreclr/commit/7611127a4f7757952ff0433ec386d8bb3d55d8f1