Register allocator: set CSRFirstUseCost to 5 for ARM64.

A value of 5 means if we have a split or spill option that has a really
low cost (1 << 14 is the entry frequency), we will choose to spill
or split the really cold path before using a callee-saved register.

This gives us the performance benefit on SPECInt2k and is also conservative.

rdar://16162005


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205248 91177308-0d34-0410-b5e6-96231b3b80d8