[X86] Remove dyn_casts to ConstantSDNode for operand 1 of...

[X86] Remove dyn_casts to ConstantSDNode for operand 1 of X86ISD::VSRLI/VSRAI/VSRLI. Use getConstantOperandVal and APInt operations.

These nodes should only ever be formed with an i8 TargetConstant
so we don't need to check for it to be a constant. It's also
always 8-bits so we don't need to use APInt compare functions.