[DAGCombiner] allow hoisting vector bitwise logic ahead of truncates

The transform performs a bitwise logic op in a wider type followed by
truncate when both inputs are truncated from the same source type:
logic_op (truncate x), (truncate y) --> truncate (logic_op x, y)

There are a bunch of other checks that should prevent doing this when 
it might be harmful.

We already do this transform for scalars in this spot. The vector 
limitation was shared with a check for the case when the operands are 
extended. I'm not sure if that limit is needed either, but that would 
be a separate patch.

Differential Revision: https://reviews.llvm.org/D55448

llvm-svn: 349303