rs6000: Fix infinite loop building ghostscript and icu [PR93658]

Fix rs6000_legitimate_address_p(), which erroneously marks a valid Altivec
address as being invalid, which causes LRA's process_address()  to go into
an infinite loop spilling the same address over and over again.

gcc/
	PR target/93658
	* config/rs6000/rs6000.c (rs6000_legitimate_address_p): Handle VSX
	vector modes.

gcc/testsuite/
	PR target/93658
	* gcc.target/powerpc/pr93658.c: New test.

gcc/testsuite/gcc.target/powerpc/pr93658.c

@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -fstack-protector-strong -mcpu=power8" } */
+/* { dg-require-effective-target powerpc_vsx_ok } */
+
+/* PR93658: Failure compiling this test is an infinite loop in LRA's
+   process_address(), so set a short timeout limit.  */
+/* { dg-timeout 5 } */
+
+void bar();
+char b;
+void
+foo (void)
+{
+  char a;
+  int d = b;
+  char *e = &a;
+  while (d)
+    *e++ = --d;
+  bar ();
+}