combineMulToPMADDWD is currently limited to legal types, but there's no reason why we can't handle any larger type that the existing SplitOpsAndApply code can use to split to legal X86ISD::VPMADDWD ops.
This also exposed a missed opportunity for pre-SSE41 targets to handle SEXT ops from types smaller than vXi16 - without PMOVSX instructions these will always be expanded to unpack+shifts, so we can cheat and convert this into a ZEXT(SEXT()) sequence to make it a valid PMADDWD op.
Nit: Should this be mul 2 directly? Then we just need to check 32 <= NumElts.
The 16 in comparison looks more like a magic number.