AMDGPU: Correctly lower llvm.exp.f32

The library expansion has too many paths for all the permutations of
DAZ, unsafe and the 3 exp functions. It's easier to expand it in the
backend when we know all of these things. The library currently misses
the no-infinity check on the overflow, which this handles optimizing
out.

Some of the <3 x half> fast tests regress due to vector widening
dropping flags which will be fixed separately.

Apparently there is no exp10 intrinsic, but there should be. Adds some
deadish code in preparation for adding one while I'm following along
with the current library expansion.