[SelectionDAGBuilder] Remove unneeded vector bitcast from visitTargetIntrinsic.

This seems to be a leftover from a long time ago when there was
an ISD::VBIT_CONVERT and a MVT::Vector. It looks like in those days
the vector type was carried in a VTSDNode.

As far as I know, these days ComputeValueTypes would have already
assigned "Result" the same type we're getting from TLI.getValueType
here. Thus the BITCAST is always a NOP. Verified by adding an assert
and running check-llvm.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D117335