[DebugInfo][InstCombine] Preserve DI after combining zext

When zext is EvaluatedInDifferentType, InstCombine
drops the dbg.value intrinsic. This patch tries to
preserve said DI, by inserting the zext's old DI in the
resulting instruction. (Only for integer type for now)

Differential Revision: https://reviews.llvm.org/D48331

llvm-svn: 336254