[llvm] [CodeGen] Simplify updateLiveIn in MachineSink (PR #79831)

[Kazushi Marukawa via llvm-commits]

kaz7 wrote:

This patch avoids the problem I mentioned.  However, it won't solve the problem, in my opinion.  We don't want to have 0x00000000 lanemask value for general registers, don't we?  #79462 correct such problems.  Please consider to apply #79462 too, please.

I think this patch itself is a good enhancement.  So, I appreciate if you review both, this and #79462.

I'll try to create a regression test for this patch too.

https://github.com/llvm/llvm-project/pull/79831