[mlir][Conversion/GPUCommon] Fix bug in conversion of `math` ops

The common GPU operation transformation that lowers `math` operations
to function calls in the `gpu-to-nvvm` and `gpu-to-rocdl` passes handles
`vector` types by applying the function to each scalar and returning a
new vector. However, there was a typo that results in incorrectly
accumulating the result vector, and the rewrite returns an `llvm.mlir.undef`
result instead of the correct vector. A patch is added and tests are
strengthened.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D154269