[CodeGen]Refactor CpuSupports/CPUIs Builtin Code Gen to better work with

"target" implementation

A small set of refactors that'll make it easier for me to implement 'target' 
support.

First, extract the CPUSupports functionality into its own function. 
THis has the advantage of not wasting time in this builtin to deal with 
arguments.
Second, pulls both CPUSupports and CPUIs implementation into a member-function, 
so that it can be called from the resolver generation that I'm working on.
Third, creates an overload that takes simply the feature/cpu name (rather than 
extracting it from a callexpr), since that info isn't available later.

Note that despite how the 'diff' looks, the EmitX86CPUSupports function simply 
takes the implementation out of the 'switch'.

llvm-svn: 312355