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

[Matt Arsenault via llvm-commits]

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@@ -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
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arsenm wrote:

These are not mutually exclusive. A register class constraint does not imply there is no LLT 

https://github.com/llvm/llvm-project/pull/115979