I observed a crash in device OpenMP lowering when compiling with
`-fdefault-integer-8`. In `targetParallelCallback`, `NumThreads` can be
`i64`, but `__kmpc_parallel_60` expects an `i32` `num_threads`
parameter, which caused a bad-signature assertion during call creation.
The fix is to use `CreateZExtOrTrunc(..., Int32)` for the `num_threads`
argument before building the runtime call. This matches the handling
used in clang in `CGOpenMPRuntimeGPU::emitParallelCall`.
The problem can be seen with the following testcase whe compiled with
`flang -fopenmp --offload-arch=gfx90a test.f90 -fdefault-integer-8``
```
program test
implicit none
integer :: nthreads
integer :: i
nthreads = 137
!$omp target teams distribute parallel do num_threads(nthreads)
do i = 1, 1
end do
!$omp end target teams distribute parallel do
end program test
```
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