[llvm] [VPlan] Give VPInstruction::ExplicitVectorLength name. NFC (PR #175493)

[Luke Lau via llvm-commits]

lukel97 wrote:

> might be good to update one of the RISCV VPlan printing tests to guard against this begin printed as expected

I think the tests already use a regex e.g. `; IF-EVL-NEXT:    EMIT-SCALAR vp<[[EVL:%.+]]> = EXPLICIT-VECTOR-LENGTH vp<[[AVL]]>`, are you suggesting to explicitly match `EMIT-SCALAR vp<evl> = ...`?

https://github.com/llvm/llvm-project/pull/175493