[DAGCombine] Allow vector constant folding of any value type before type legalization

The patch comes in 2 parts:

1 - it makes use of the SelectionDAG::NewNodesMustHaveLegalTypes flag to tell when it can safely constant fold illegal types.

2 - it correctly resets SelectionDAG::NewNodesMustHaveLegalTypes at the start of each call to SelectionDAGISel::CodeGenAndEmitDAG so all the pre-legalization stages can make use of it - not just the first basic block that gets handled.

Fix for PR30760

Differential Revision: https://reviews.llvm.org/D29568

llvm-svn: 294749