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

[Daniel Sanders 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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dsandersllvm wrote:

That's true but I'm just moving the existing code in this PR so that we check the LLT is valid before asserting on properties of it. Do you want me to fix that in the same PR?

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