[MCA] Remove SM.hasNext() call in FetchStage::execute.

Summary:
This is redundant, as FetchStage::getNextInstruction already checks this
and returns llvm::ErrorSuccess() as appropriate.

NFC.

Reviewers: andreadb

Subscribers: gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D52642

llvm-svn: 343555