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

[Daniel Sanders via llvm-commits]

dsandersllvm wrote:

Thanks, the test that failed for me was TEST_F(AArch64GISelMITest, TestKnownBitsCstWithClass) in llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp but it only seems to fail in our downstream fork. I haven't quite figured out why yet but both versions have isValid() false while having different values internally to the LLT.

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