Fix a DAG combiner assertion failure when constant folding BUILD_VECTORS.

Fix a DAG combiner assertion failure when constant folding BUILD_VECTORS.

svn r139159 caused SelectionDAG::getConstant() to promote BUILD_VECTOR operands
with illegal types, even before type legalization.  For this testcase, that led
to one BUILD_VECTOR with i16 operands and another with promoted i32 operands,
which triggered the assertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142370 91177308-0d34-0410-b5e6-96231b3b80d8