X86Vector: relax checks in rsqrt's integration test

Instead of hard-coding results for both Intel and AMD, let's relax
the checks to simplify the test while supporting both implementations.
Note that:
- If a new hardware implementation comes up in the future, it is likely
  to pass the relaxed tests, i.e. no future maintenance burden for us.
- If something terribly wrong happens (e.g. instead of rsqrt we
  execute 1/sqrt), the tests will probably catch it, since the relaxed
  tests expect low precision (e.g. rsqrt(1) != 1.0).

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D111461