VPBlendRecipe does not use the first mask operand. Removing it allows
VPlan-based DCE to remove unused mask computations.
This also fixes#87410, where unused Not VPInstructions are considered
having only their first lane demanded, but some of their operands
providing a vector value due to other users.
Fixes https://github.com/llvm/llvm-project/issues/87410
PR: https://github.com/llvm/llvm-project/pull/87770
c836983671