[llvm] [RegAllocFast] Handle single-vdef instrs faster (PR #96284)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

> I'm wondering, how many instructions besides inlineasm actually need this analysis.

I know of a couple cases that have 2 tied vreg defs 

https://github.com/llvm/llvm-project/pull/96284