SpeculativeExecution: fixed ingoring free execution

Summary:
After updating cost model in AMDGPU target (47a5c36b) the pass started to
ignore some BBs since they got all instructions estimated as free.

Reviewers: arsenm, chandlerc, nhaehnle

Reviewed By: nhaehnle

Subscribers: jvesely, wdng, nhaehnle, tpr, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74825