[SPIRV] Fix type mismatch assertion in insertvalue. (#143131)
The code was incorrectly converting all `undef` arguments to `i32`,
while the `spv_insertv` intrinsics only expects that for the first
operand, representing the aggregate type.
Fixes https://github.com/llvm/llvm-project/issues/127977
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Co-authored-by: Michal Paszkowski <michal@michalpaszkowski.com>