[VPlan] Set flags when constructing zexts using VPWidenCastRecipe (#164198)

createWidenCast doesn't set the flag type, so when we simplify trunc
(zext nneg x) -> zext x we would hit an assertion in CSE that the flag
types don't match with other VPWidenCastRecipes that weren't simplified.

This fixes it the same way trunc flags are handled too.

As an aside I think it should be correct to preserve the nneg flag in
this case since the input operand is still non-negative after the
transform. But that's left to another PR.

Fixes https://github.com/llvm/llvm-project/issues/164171

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