[FastISel][AArch64] Factor out scale factor calculation. NFC.

Factor out the code that determines the implicit scale factor of memory
operations for a given value type.
[FastISel][AArch64] Fold sign-/zero-extends into the load instruction.

The sign-/zero-extension of the loaded value can be performed by the memory
instruction for free. If the result of the load has only one use and the use is
a sign-/zero-extend, then we emit the proper load instruction. The extend is
only a register copy and will be optimized away later on.

Other instructions that consume the sign-/zero-extended value are also made
aware of this fact, so they don't fold the extend too.

This fixes rdar://problem/18495928.