[SPIR-V] Fix OpVectorShuffle undef emission (#151993)
When an undef/poison value is lowered as a an immediate, it becomes -1.
When reaching the backend, the -1 was printed as operand to
OpVectorShuffle instead of the proper 0xFFFFFFFF.
From the SPIR-V spec:
A Component literal may also be FFFFFFFF, which means the
corresponding result component has no source and is undefined.
The reason the existing tests were passing `spirv-val` was because the
binary format was used as output, meaning the `-1` was lowered to
`0xFFFFFFFF`. But when the text format is used, `-1` is emitted as-is
which is wrong.
Fixes #151691
