rs6000: Fix -mpower9-vector -mno-altivec ICE (PR87560)

PR87560 reports an ICE when a test case is compiled with -mpower9-vector
and -mno-altivec.  This patch terminates compilation with an error when
this combination (and other unreasonable ones) are requested.

Bootstrapped and tested on powerpc64le-unknown-linux-gnu with no
regressions.  Reported error is now:

f951: Error: '-mno-altivec' turns off '-mpower9-vector'

2020-03-02  Bill Schmidt  <wschmidt@linux.ibm.com>

	PR target/87560
	* rs6000-cpus.def (OTHER_ALTIVEC_MASKS): New #define.
	* rs6000.c (rs6000_disable_incompatible_switches): Add table entry
	for OPTION_MASK_ALTIVEC.

gcc/ChangeLog

+2020-03-04  Bill Schmidt  <wschmidt@linux.ibm.com>
+
+	PR target/87560
+	* rs6000-cpus.def (OTHER_ALTIVEC_MASKS): New #define.
+	* rs6000.c (rs6000_disable_incompatible_switches): Add table entry
+	for OPTION_MASK_ALTIVEC.
+
 2020-03-04  Jakub Jelinek  <jakub@redhat.com>
 
 	PR debug/93888

gcc/config/rs6000/rs6000-cpus.def

@@ -101,6 +101,10 @@
 				 | OPTION_MASK_FLOAT128_KEYWORD		\
 				 | OPTION_MASK_P8_VECTOR)
 
+/* Flags that need to be turned off if -mno-altivec.  */
+#define OTHER_ALTIVEC_MASKS	(OTHER_VSX_VECTOR_MASKS			\
+				 | OPTION_MASK_VSX)
+
 #define POWERPC_7400_MASK	(OPTION_MASK_PPC_GFXOPT | OPTION_MASK_ALTIVEC)
 
 /* Deal with ports that do not have -mstrict-align.  */

gcc/config/rs6000/rs6000.c

@@ -23632,6 +23632,7 @@ rs6000_disable_incompatible_switches (void)
     { OPTION_MASK_P9_VECTOR,	OTHER_P9_VECTOR_MASKS,	"power9-vector"	},
     { OPTION_MASK_P8_VECTOR,	OTHER_P8_VECTOR_MASKS,	"power8-vector"	},
     { OPTION_MASK_VSX,		OTHER_VSX_VECTOR_MASKS,	"vsx"		},
+    { OPTION_MASK_ALTIVEC,	OTHER_ALTIVEC_MASKS,	"altivec"	},
   };
 
   for (i = 0; i < ARRAY_SIZE (flags); i++)