Handle ConstantExpr correctly in SelectionDAGBuilder

This change fixes a bug in SelectionDAGBuilder::visitInsertValue and SelectionDAGBuilder::visitExtractValue where constant expressions (InsertValueConstantExpr and ExtractValueConstantExpr) would be treated as non-constant instructions (InsertValueInst and ExtractValueInst). This bug resulted in an incorrect memory access, which manifested as an assertion failure in SDValue::SDValue.

Fixes PR#33094.

Submitted on behalf of @Praetonus (Benoit Vey)

Differential Revision: https://reviews.llvm.org/D34538

llvm-svn: 307502