Allow tree-ssa.exp to be run by itself

tree-ssa testcases sometimes check autovect effective target
but does not set it up.  On MIPS, those testcases fail with
some TCL error messages.  This fixes the issue by calling
check_vect_support_and_set_flags inside tree-ssa.exp.
There might be other .exp files which need to be done this
way too but I have not checked all of them.

Tested on x86_64-linux-gnu and a cross to mips64-octeon-linux-gnu.
Both full run of the testsuite and running tree-ssa.exp by itself.

testsuite/ChangeLog:
* tree-ssa.exp: Set DEFAULT_VECTCFLAGS and DEFAULT_VECTCFLAGS.
Call check_vect_support_and_set_flags also.

gcc/testsuite/ChangeLog

+2020-01-22  Andrew Pinski  <apinski@marvell.com>
+
+	* tree-ssa.exp: Set DEFAULT_VECTCFLAGS and DEFAULT_VECTCFLAGS.
+	Call check_vect_support_and_set_flags also.
+
 2020-01-22  David Malcolm  <dmalcolm@redhat.com>
 
 	PR analyzer/93378

gcc/testsuite/gcc.dg/tree-ssa/tree-ssa.exp

@@ -19,6 +19,17 @@
 # Load support procs.
 load_lib gcc-dg.exp
 
+# Set up flags used for tests that don't specify options.
+global DEFAULT_VECTCFLAGS
+set DEFAULT_VECTCFLAGS ""
+
+# Set up a list of effective targets to run vector tests for all supported
+# targets.
+global EFFECTIVE_TARGETS
+set EFFECTIVE_TARGETS ""
+
+check_vect_support_and_set_flags
+
 # If a testcase doesn't have special options, use these.
 global DEFAULT_CFLAGS
 if ![info exists DEFAULT_CFLAGS] then {