1) Change the interface to TargetLowering::ExpandOperationResult to 
   take and return entire NODES that need a result expanded, not just
   the value.  This allows us to handle things like READCYCLECOUNTER,
   which returns two values.
2) Implement (extremely limited) support in LegalizeDAG::ExpandOp for MERGE_VALUES.
3) Reimplement custom lowering in LegalizeDAGTypes in terms of the new
   ExpandOperationResult.  This makes the result simpler and fully 
   general.
4) Implement (fully general) expand support for MERGE_VALUES in LegalizeDAGTypes.
5) Implement ExpandOperationResult support for ARM f64->i64 bitconvert and ARM
   i64 shifts, allowing them to work with LegalizeDAGTypes.
6) Implement ExpandOperationResult support for X86 READCYCLECOUNTER and FP_TO_SINT,
   allowing them to work with LegalizeDAGTypes.

LegalizeDAGTypes now passes several more X86 codegen tests when enabled and when
type legalization in LegalizeDAG is ifdef'd out.

llvm-svn: 44300