[llvm] [Intrinsics][PreISelIntrinsicLowering] llvm.memset.inline length no longer needs to be constant (PR #95397)

[Alex Bradbury via llvm-commits]

asb wrote:

@nikic just wanted to get another LGTM for making this change to the intrinsic limitations (which I'd intend to apply across the other inline mem* intrinsics in future PRs).

https://github.com/llvm/llvm-project/pull/95397