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

[Alexis Engelke via llvm-commits]

aengelke wrote:

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

I was wondering in how many cases we actually hit a `ClassSize < RegClassDefCounts[RC]` case -- so multiple virtual defs *and* a possibly-too-small regclass.

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