Make ISEL used internal functions const/nothrow where appropriate

Both .VEC_SET and .VEC_EXTACT and the various .VCOND internal functions
are operating on registers only and they are not supposed to raise
any exceptions.  The following makes them const/nothrow.  I've
verified this avoids useless SSA updates in ISEL.

	* internal-fn.def (VCOND, VCONDU, VCONDEQ, VCOND_MASK,
	VEC_SET, VEC_EXTRACT): Make ECF_CONST | ECF_NOTHROW.