This patch updates the translation of `omp.wsloop` with a nested
`omp.simd` to prevent uses of block arguments defined by the latter from
triggering null pointer dereferences.
This happens because the inner `omp.simd` operation representing
composite `do simd` constructs is currently skipped and not translated,
but this results in block arguments defined by it not being mapped to an
LLVM value. The proposed solution is to map these block arguments to the
LLVM value associated to the corresponding operand, which is defined
above.
a1f2fb6078