WidenVecRes_BUILD_VECTOR must use the first operand's type

WidenVecRes_BUILD_VECTOR must use the first operand's type

Because integer BUILD_VECTOR operands may have a larger type than the result's
vector element type, and all operands must have the same type, when widening a
BUILD_VECTOR node by adding UNDEFs, we cannot use the vector element type, but
rather must use the type of the existing operands.

Another bug found by llvm-stress.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185960 91177308-0d34-0410-b5e6-96231b3b80d8