Revert some auto-generated checks where a wildcard was needed

This change reverts a batch of autogenerated checks in the pow-exp.ll
file where a previous version had used wildcards to account for minor
variations in constant folding by different math libraries. The test is
also updated to add a comment that will prevent future autogeneration.

See: https://discourse.llvm.org/t/auto-generated-checks-with-inexact-fp-results/71713

Differential Revision: https://reviews.llvm.org/D158758