[ValueTracking, VectorUtils] Refactor getIntrinsicIDForCall

The functionality contained within getIntrinsicIDForCall is two-fold: it
checks if a CallInst's callee is a vectorizable intrinsic.  If it isn't
an intrinsic, it attempts to map the call's target to a suitable
intrinsic.

Move the mapping functionality into getIntrinsicForCallSite and rename
getIntrinsicIDForCall to getVectorIntrinsicIDForCall while
reimplementing it in terms of getIntrinsicForCallSite.

llvm-svn: 266801