GlobalISel: Don't assert on MoreElements creating vectors

[arsenm]

Summary

If the original type was a scalar, it should be valid to add elements
to turn it into a vector.

Tests included with following legalization change.

[dsanders]

LGTM

[arsenm]

fba1fbb9c7367990a0561a36dbf600fc51847246

llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp

if (Rule.getAction() == Custom)
     return true;
 
   const unsigned TypeIdx = Mutation.first;
   const LLT OldTy = Q.Types[TypeIdx];
   const LLT NewTy = Mutation.second;
 
   switch (Rule.getAction()) {
   case FewerElements:
-  case MoreElements: {
     if (!OldTy.isVector())
       return false;
-
+    LLVM_FALLTHROUGH;
+  case MoreElements: {
+    // MoreElements can go from scalar to vector.
+    const unsigned OldElts = OldTy.isVector() ? OldTy.getNumElements() : 1;
     if (NewTy.isVector()) {
       if (Rule.getAction() == FewerElements) {
         // Make sure the element count really decreased.
-        if (NewTy.getNumElements() >= OldTy.getNumElements())
+        if (NewTy.getNumElements() >= OldElts)
           return false;
       } else {
         // Make sure the element count really increased.
-        if (NewTy.getNumElements() <= OldTy.getNumElements())
+        if (NewTy.getNumElements() <= OldElts)
           return false;
       }
     }
 
     // Make sure the element type didn't change.
-    return NewTy.getScalarType() == OldTy.getElementType();
+    return NewTy.getScalarType() == OldTy.getScalarType();
   }
   case NarrowScalar:
   case WidenScalar: {
     if (OldTy.isVector()) {
       // Number of elements should not change.
       if (!NewTy.isVector() || OldTy.getNumElements() != NewTy.getNumElements())
         return false;
     } else {