[llvm-exegesis][NFC] Rename ExegesisTarget::decrementLoopCounterAndLoop()

Summary: To decrementLoopCounterAndJump, and explicitely take the jump target.

Reviewers: gchatelet

Subscribers: tschuett, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68375

llvm-svn: 373571