Fixes for Zero Offset field sequence tracking

Fixes for Zero Offset field sequence tracking
 - A GT_LCL_VAR may have a zeroOffset field
 - Add an assert to prevent building field sequences with duplicates
 - Fix fgMorphField when we have a zero offset field
Improve fgAddFieldSeqForZeroOffset
 - Add JItDump info
 - Handle GT_LCL_FLD

Changing the sign of an int constant also remove any field sequence information.

Added method header comment for fgAddFieldSeqForZeroOffset

Changed when we call fgAddFieldSeqForZeroOffset to be before the call to fgMorphSmpOp.

Prefer calling fgAddFieldSeqForZeroOffset() to GetZeroOffsetFieldMap()->Set()