[DAGCombiner] In visitBITCAST when trying to constant fold the bitcast, only...

[DAGCombiner] In visitBITCAST when trying to constant fold the bitcast, only call getBitcast if its an fp->int or int->fp conversion even when before legalize ops.

Previously if !LegalOperations we would blindly call getBitcast and hope that getNode would constant fold it. But if the conversion is between a vector and a scalar, getNode has no simplification.

This means we would just get back the original N. We would then return that N which would make the caller of visitBITCAST think that we used CombineTo and did our own worklist management. This prevents target specific optimizations from being called for vector/scalar bitcasts until after legal operations.

llvm-svn: 331896