[VPlan] Move emission of \\l\"+\n to dumpBasicBlock (NFC).

The patch standardizes printing of VPRecipes a bit, by hoisting out the
common emission of \\l\"+\n. It simplifies the code and is also a first
step towards untangling printing from DOT format output, with the goal
of making the DOT output optional and to provide a more concise debug
output if DOT output is disabled.

Reviewers: gilr, Ayal, rengolin

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D78883