[llvm] [VPlan] Only store RecurKind + FastMathFlags in VPReductionRecipe. NFCI (PR #131300)

[Luke Lau via llvm-commits]

lukel97 wrote:

> There's a new use of the recurrence descriptor added in #113903 by @ElvisWang123 . Would be good to check if RecurrenceKind is enough for that patch as well

I just took a look there, #113903 should be fine since it's just using the opcode

https://github.com/llvm/llvm-project/pull/131300