Add VerifyNode, a place to put sanity checks on

Add VerifyNode, a place to put sanity checks on
generic SDNode's (nodes with their own constructors
should do sanity checking in the constructor).  Add
sanity checks for BUILD_VECTOR and fix all the places
that were producing bogus BUILD_VECTORs, as found by
"make check".  My favorite is the BUILD_VECTOR with
only two operands that was being used to build a
vector with four elements!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53850 91177308-0d34-0410-b5e6-96231b3b80d8