[multiversion] Switch the TTI queries from TargetMachine to Subtarget

[multiversion] Switch the TTI queries from TargetMachine to Subtarget
now that we have a correct and cached subtarget specific to the
function.

Also, finish providing a cached per-function subtarget in the core
LLVMTargetMachine -- that layer hadn't switched over yet.

The only use of the TargetMachine was to re-lookup a subtarget for
a particular function to work around the fact that TTI was immutable.
Now that it is per-function and we haved a cached subtarget, use it.

This still leaves a few interfaces with real warts on them where we were
passing Function objects through the TTI interface. I'll remove these
and clean their usage up in subsequent commits now that this isn't
necessary.

llvm-svn: 227738