[Backport to 17] Fix SPIRVCopyMemory::validate() (KhronosGroup#2770)

The asserts should be checking the `Target` member variable; `Id` is
not used for this class.

Only fix the wrong asserts for now; proper handling and testing of
`OpCopyMemory` will be done in a followup commit.

Contributes to KhronosGroup#2769

(cherry picked from commit 9d2926d)

lib/SPIRV/libSPIRV/SPIRVInstruction.h

@@ -2100,10 +2100,15 @@ class SPIRVCopyMemory : public SPIRVInstruction, public SPIRVMemoryAccess {
   }
 
   void validate() const override {
-    assert((getValueType(Id) == getValueType(Source)) && "Inconsistent type");
-    assert(getValueType(Id)->isTypePointer() && "Invalid type");
-    assert(!(getValueType(Id)->getPointerElementType()->isTypeVoid()) &&
-           "Invalid type");
+    assert(getValueType(Target)->isTypePointer() && "Invalid Target type");
+    assert(getValueType(Source)->isTypePointer() && "Invalid Source type");
+    assert(!(getValueType(Target)->getPointerElementType()->isTypeVoid()) &&
+           "Invalid Target element type");
+    assert(!(getValueType(Source)->getPointerElementType()->isTypeVoid()) &&
+           "Invalid Source element type");
+    assert(getValueType(Target)->getPointerElementType() ==
+               getValueType(Source)->getPointerElementType() &&
+           "Mismatching Target and Source element types");
     SPIRVInstruction::validate();
   }