[SIL] Add tests for 'undef' values & code fixes

This change validates that 'undef' can appear in most places where values are
expected by the SIL parser. Fixes are also included for the 'select_value'
instruction. This resolves SR-304.

Author: Emanuel Zephir

lib/SIL/SILVerifier.cpp

@@ -2546,15 +2546,18 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
       SILValue casevalue;
       SILValue result;
       std::tie(casevalue, result) = I->getCase(i);
-      auto  *il = dyn_cast<IntegerLiteralInst>(casevalue);
-      require(il,
-              "select_value case operands should refer to integer literals");
-      APInt elt = il->getValue();
+      
+      if (!isa<SILUndef>(casevalue)) {
+        auto  *il = dyn_cast<IntegerLiteralInst>(casevalue);
+        require(il,
+                "select_value case operands should refer to integer literals");
+        APInt elt = il->getValue();
 
-      require(!seenCaseValues.count(elt),
-              "select_value dispatches on same case value more than once");
+        require(!seenCaseValues.count(elt),
+                "select_value dispatches on same case value more than once");
 
-      seenCaseValues.insert(elt);
+        seenCaseValues.insert(elt);
+      }
 
       requireSameType(I->getOperand()->getType(), casevalue->getType(),
                       "select_value case value must match type of operand");