[CodeGenPrep]No negative cost in the ExtLd promotion

Summary: This change prevent the signed value of cost from being negative as the value is passed as an unsigned argument.

Reviewers: mcrosier, jmolloy, qcolombet, javed.absar

Reviewed By: mcrosier, qcolombet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28871

llvm-svn: 293307