Fix inconsistent handling of zero extending casts

Fix inconsistent handling of zero extending casts

For casts that are supposed to zero extend the GTF_UNSIGNED must always be set (and obeyed).

Some code failed to set the flag (e.g. when importing add.ovf.un instructions having native int and int32 operands) and some other code failed to check the flag (e.g. x64 codegen, gtFoldExprConst) and instead decided to zero extend based on the cast destination type.

This resulted in discrepancies between ARM64 and x64 codegen and between constant folding performed by gtFoldExprConst and VN's EvalCastForConstantArgs.