[llvm] [CodeGen] Remove redundant checks (PR #99524)

[Craig Topper via llvm-commits]

topperc wrote:

> It turns out we can safely use DAG.computeKnownBits(N0).countMinLeadingZeros() with constant legal vectors, so remove the check for it.

It is safe, but you have to write tests for it.

https://github.com/llvm/llvm-project/pull/99524