[X86] Don't check for VBROADCAST_LOAD being a user of the source of a...

[X86] Don't check for VBROADCAST_LOAD being a user of the source of a VBROADCAST when trying to share broadcasts.

The only things VBROADCAST_LOAD uses is an address and a chain
node. It has no vector inputs.

So if its a user of the source of another broadcast that could
only mean one of two things. The other broadcast is broadcasting
the address of the broadcast_load. Or the source is a load and
the use we're seeing is the chain result from that load. Neither
of these cases make sense to combine here.

This issue was reported post-commit r373871. Test case has not
been reduced yet.

llvm-svn: 374862