[rs6000] int->machine_mode in rs6000-c.c

2017-08-30  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/rs6000/rs6000-c.c (altivec_resolve_overloaded_builtin):
	Use machine_mode rather than int for arg1_mode.

From-SVN: r251447