[IndVars] Remove unreasonable checks in rewriteLoopExitValues

A piece of logic in rewriteLoopExitValues has a weird check on number of
users which allowed an unprofitable transform in case if an instruction has
more than 6 users.

Differential Revision: https://reviews.llvm.org/D51404
Reviewed By: etherzhhb

llvm-svn: 342444