[RegAllocGreedy][Last Chance Recoloring] Change the name of the exhaustive search option.
fexhaustive-register-search => exhaustive-register-search
'f' is a Clang thing!

This is related to PR18747.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206075 91177308-0d34-0410-b5e6-96231b3b80d8