[InlineCost] Remove visitUnaryInstruction()

The simplifyInstruction() in visitUnaryInstruction() does not trigger
for all of check-llvm. Looking at all delegates to UnaryInstruction in
InstVisitor, the only instructions that either don't have a visitor in
CallAnalyzer, or redirect to UnaryInstruction, are VAArgInst and Alloca.
VAArgInst will never get simplified, and visitUnaryInstruction(Alloca)
would always return false anyway.

Reviewed By: mtrofin, lebedev.ri

Differential Revision: https://reviews.llvm.org/D101577