[VPlan] Helper to check if a recipe uses scalar values of op.

This patch adds a helper to check if a recipe only uses scalars of a
given operand. This is similar to onlyFirstLaneUsed, which was
introduced earlier.

By default, usesScalars falls back on onlyFirstLaneUsed.

Will be used by D120828.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D120827