[SelectionDAGBuilder] Use getConstant instead of getTargetConstant to build...

[SelectionDAGBuilder] Use getConstant instead of getTargetConstant to build the offset for struct types in getUniformBase.

getTargetConstant prevents any optimizations from operating on the
value and basically says its already been iseled. But since we
want the index to be in a register, this isn't true.

Prior to this we were generating a vbroadcast with an immediate
argument which is illegal and was flagged by the expensive checks
bot.