[GlobalISel] Move DemandedElt's APInt size assert after isValid() check (#115979)

This prevents the assertion from wrongly triggering on invalid LLT's

Author: dsandersllvm

llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp

@@ -147,6 +147,15 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
   unsigned Opcode = MI.getOpcode();
   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.
+  if (!DstTy.isValid()) {
+    Known = KnownBits();
+    return;
+  }
+
 #ifndef NDEBUG
   if (DstTy.isFixedVector()) {
     assert(
@@ -158,15 +167,6 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
   }
 #endif
 
-  // 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.
-  if (!DstTy.isValid()) {
-    Known = KnownBits();
-    return;
-  }
-
   unsigned BitWidth = DstTy.getScalarSizeInBits();
   auto CacheEntry = ComputeKnownBitsCache.find(R);
   if (CacheEntry != ComputeKnownBitsCache.end()) {