[VPlan] Remove unused first mask op from VPBlendRecipe. (#87770)

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