[OpenCL][Sema] Minor refactoring and constraint checking

[mantognini]

Summary

Simplify code a bit and add assertion to address post-landing comments
from D64083.

[mantognini]

This should address the minor refactoring requesting in D64083.

[rjmccall]

Thanks.

cfe/trunk/lib/Sema/SemaExprCXX.cpp

// Case 2.  _Complex x -> y
         From = ImpCastExprToType(From, ToType,
                    isFloatingComplex ? CK_FloatingToIntegral : CK_IntegralCast,
                                  VK_RValue, /*BasePath=*/nullptr, CCK).get();
       }
     }
     break;
 
   case ICK_Block_Pointer_Conversion: {
-    QualType LHSType = Context.getCanonicalType(ToType).getUnqualifiedType();
-    QualType RHSType = Context.getCanonicalType(FromType).getUnqualifiedType();
-
-    // Assumptions based on Sema::IsBlockPointerConversion.
-    assert(isa<BlockPointerType>(LHSType) && "BlockPointerType expected");
-    assert(isa<BlockPointerType>(RHSType) && "BlockPointerType expected");
-
     LangAS AddrSpaceL =
-        LHSType->getAs<BlockPointerType>()->getPointeeType().getAddressSpace();
+        ToType->castAs<BlockPointerType>()->getPointeeType().getAddressSpace();
     LangAS AddrSpaceR =
-        RHSType->getAs<BlockPointerType>()->getPointeeType().getAddressSpace();
+        FromType->castAs<BlockPointerType>()->getPointeeType().getAddressSpace();
+    assert(Qualifiers::isAddressSpaceSupersetOf(AddrSpaceL, AddrSpaceR) &&
+           "Invalid cast");
     CastKind Kind =
         AddrSpaceL != AddrSpaceR ? CK_AddressSpaceConversion : CK_BitCast;
     From = ImpCastExprToType(From, ToType.getUnqualifiedType(), Kind,
                              VK_RValue, /*BasePath=*/nullptr, CCK).get();
     break;
   }
 
   case ICK_TransparentUnionConversion: {