Don't undef result operand if there is none

The mod_by_zero and negative_shift helper may also be used by
ASSIGN_OP, in which case there is not necessarily a result operand.
If the s

Don't undef result operand if there is none

The mod_by_zero and negative_shift helper may also be used by
ASSIGN_OP, in which case there is not necessarily a result operand.
If the stars aligned just right, this used to clobber other parts
of the call frame.

For these two helpers, check whether the result_type is TMP/VAR
before setting to UNDEF:

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