X86 cost model: Vector shifts are expensive in most cases

The default logic does not correctly identify costs of casts because they are
marked as custom on x86.

For some cases, where the shift amount is a scalar we would be able to generate
better code. Unfortunately, when this is the case the value (the splat) will get
hoisted out of the loop, thereby making it invisible to ISel.

radar://13130673
radar://13537826