Tighten description of ISD::BUILD_VECTOR

VerifySDNode(...) in SelectionDAG.cpp shows that the operands of a BUILD_VECTOR must all be the same type. This patch cleans up the comment in ISDOpcodes.h to make that more obvious. Also, remove the requirement that the number of elements must be a power-of-2. That's not true.

Differential Revision: https://reviews.llvm.org/D83413