[InstCombine] Fix errno bug in pow expansion to sqrt

A conversion from `pow` to `sqrt` shall not call an `errno`-setting
`sqrt` with -//infinity//: the `sqrt` will set `EDOM` where the `pow`
call need not.

This patch avoids the erroneous (pun not intended) transformation by
applying the restrictions discussed in the thread for
https://lists.llvm.org/pipermail/llvm-dev/2020-September/145051.html.

The existing tests are updated (depending on emphasis in the checks for
library calls, avoidance of overlap, and overall coverage):
  - to add `ninf`, retaining the intended library call,
  - to use the intrinsic, retaining the use of `select`, or
  - to expect the replacement to not occur.

The following is tested:
  - The pow intrinsic folds to a `select` instruction to
    handle -//infinity//.
  - The pow library call folds, with `ninf`, to `sqrt` without the
    `select` instruction associated with handling -//infinity//.
  - The pow library call does not fold to `sqrt` without `ninf`.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D87877