[TTI] Improved cost heuristic for cttz/ctlz calls.

This patch is a follow-up of r228826 (see code-review: D7506).

Now that SimplifyCFG uses TargetTransformInfo for cost analysis, we 
have to fix the cost heuristic for intrinsic calls to cttz/ctlz.

This patch defines method 'getIntrinsicCost' in BasicTTIImpl: now, BasicTTIImpl
queries TLI to check if a call to cttz/ctlz is cheap for the target.

Added test cases in Transforms/SimplifyCFG/X86 to verify that on x86,
SimplifyCFG only speculates a call to cttz/ctlz if it is cheap.

Differential Revision: http://reviews.llvm.org/D7554

llvm-svn: 228829