InstCombine: Fix testing of pow libcall in errno case (#94772)

There were some tests in this file with "noerrno" in the name, but all
the tests were no errno since all the libcalls were declared with
memory(none). Ensure we have adequate coverage for the errno and
no-errno cases by duplicating the libcall transform cases into errno and
non-errno versions with callsite attributes.

Author: arsenm