[GlobalISel][AArch64] Select G_FEXP

This teaches the legalizer to handle G_FEXP in AArch64. As a result, it also
allows us to select G_FEXP.

It...

- Updates the legalizer-info tests
- Adds a test for legalizing exp
- Updates the existing fp tests to show that we can now select G_FEXP

https://reviews.llvm.org/D57483

llvm-svn: 352692