[OpenACC] Remove unnecessary uses of `getResult`, fix cast tests (#161526)
A previous review comment pointed out that operations with only a single
result implicitly convert to `mlir::Value`. This patch removes the
explicit use of `getResult` where it is unnecessary in OpenACC lowering.
However, there ARE a few cases where it is necessary where the
`mlir::ValueRange` implicit constructor from a single value is being
used, so those are untouched.
Additionally, while the previous patch was being committed (#161382), a
second patch (#161431) changed the format of cir.casts, so this patch
fixes the additional test lines for that as well.