Don't change LCL_FLD node type when replacing param with its shadow copy (#20900)

Don't change LCL_FLD node type when replacing param with its shadow copy (#20900)

Fix an issue that is reproduced when:

1) a method requires GSSecurityCookie (e.g. when allocating unsafe buffer using localloc) and

2) one of the method parameters is "vulnerable" and requires a shadow variable to be created and used instead of the original parameter variable and

3) the method parameter has small signed integer type (i.e. int8 or int16) and

4) the value of the parameter is converted to unsigned type (e.g. uint8 or uint16).