[NVPTX] Fix illegal combineInsertEltToShuffle pattern (#198259)
Adds a condition to the `isShuffleMaskLegal` method to prevent
`combineInsertEltToShuffle` from creating an illegal `vector_shuffle`
after type legalization which leads to a crash.
Context:
This is triggered when bitcasting a `v2f16` into a vector that type
legalizes to a `v2i32`. This happens on architectures supporting packed
`f32` operations (>= `sm_100`). In certain cases, this leads to
`combineInsertEltToShuffle` creating a vector shuffle with `v4f16` for
simplification. Since this happens after type legalization and `v4f16`
is an illegal type, it leads to a crash.
