[X86] Add DAG combine to turn (bitcast (vbroadcast_load)) into just a...

[X86] Add DAG combine to turn (bitcast (vbroadcast_load)) into just a vbroadcast_load if the scalar size is the same.

This improves broadcast load folding of i64 elements on 32-bit
targets where i64 isn't legal.

Previously we had to represent these as vXf64 vbroadcast_loads and
a bitcast to vXi64. But we didn't have any isel patterns
looking for that.

This also allows us to remove or simplify some isel patterns that
were looking for bitcasted vbroadcast_loads.

llvm-svn: 373566