LoopVectorize/test: clean up reduction.ll; generate using UTC (NFC) (llvm#68890)

The test reduction.ll was introduced before utils/update_test_checks.py,
and hence contains hand-written CHECK lines. Revisit the test today, and
modernize it by:

- Removing extranous attributes on functions and their arguments, as
LoopVectorize doesn't even look at these attributes.
- Removing the target datalayout, as it is not essential for
LoopVectorize.

Finally, regenerate the CHECK lines using update_test_checks.py,
eliminating hand-written error-prone CHECK lines.

Author: artagnon