[TTI] Refine the cost of EXT in getUserCost()

Now, getUserCost() only checks the src and dst types of EXT to decide it is free
or not. This change first checks the types, then calls isExtFreeImpl(), and
check if EXT can form ExtLoad at last. Currently, only AArch64 has customized
implementation of isExtFreeImpl() to check if EXT can be folded into its use.

Differential Revision: https://reviews.llvm.org/D34458

llvm-svn: 308076