S12Z: gas: Permit "extend" instructions which don't actually extend.

S12Z: gas: Permit "extend" instructions which don't actually extend.

Other assemblers permit "extending" a register into a register of a
smaller size or the same size.  It doesn't make much sense to do this
but would appear to be a valid instruction.  So change the error to a
warning.

gas/
	* config/tc-s12z.c (tfr): Change as_bad to as_warn.
	Also fix message typo and semantics.