[GlobalISel] Correct comment about type vs register class

Type and register class aren't mutually exclusive in gMIR but target
instructions don't have types on their operands that have register classes.

Author: dsandersllvm

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

@@ -148,9 +148,10 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
   LLT DstTy = MRI.getType(R);
 
   // Handle the case where this is called on a register that does not have a
-  // type constraint (i.e. it has a register class constraint instead). This is
-  // unlikely to occur except by looking through copies but it is possible for
-  // the initial register being queried to be in this state.
+  // type constraint (i.e. it's a target instruction with a register class
+  // constraint instead). This is unlikely to occur except by looking through
+  // copies but it is possible for the initial register being queried to be in
+  // this state.
   if (!DstTy.isValid()) {
     Known = KnownBits();
     return;