[AMDGPU] Make fexp.ll test autogenerated

This test is very verbose and appears generated by a script.
Make it truly autogenerated for easy updates.
NFC

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D109530

Change-Id: I1352b17b6d13ab9c5650dbe95ef0da97f71f1930