rs6000: Remove useless copy_rtx in rs6000_emit_set_{,long}_const

Function rs6000_emit_set_const/rs6000_emit_set_long_const are only invoked from
two "define_split"s where the target operand is limited to gpc_reg_operand or
int_reg_operand, then the operand must be REG_P.
And in rs6000_emit_set_const/rs6000_emit_set_long_const, to create temp rtx,
it is using code like "gen_reg_rtx({S|D}Imode)", it must also be REG_P.
So, copy_rtx is not needed for temp and dest.

This patch removes those "copy_rtx" for rs6000_emit_set_const and
rs6000_emit_set_long_const.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_emit_set_const): Remove copy_rtx.
	(rs6000_emit_set_long_const): Likewise.