[SPIR-V] Emit spv_undef intrinsic for aggregate undef operands

This change adds a new spv_undef intrinsic which is emitted in place of
aggregate undef operands and later selected to single OpUndef SPIR-V
instruction. The behavior matches that of Khronos SPIR-V Translator and
should support nested aggregates.

Differential Revision: https://reviews.llvm.org/D143107