[TableGen] Teach tablegen to allow SDNPCommutable nodes with more than 2 operands.

Summary:
Tablegen already supports commutable instrinsics with more than 2 operands. There it just assumes the first two operands are commutable.

I plan to use this to improve the generation of FMA patterns in the X86 backend.

Reviewers: aymanmus, zvi, RKSimon, spatel, arsenm

Reviewed By: arsenm

Subscribers: arsenm, llvm-commits

Differential Revision: https://reviews.llvm.org/D37430

llvm-svn: 312464