[GlobalISel][AArch64] Add instruction selection support for @llvm.log2

This teaches GlobalISel to emit a RTLib call for @llvm.log2 when it encounters
it.

It updates the existing floating point tests to show that we don't fall back on
the intrinsic, and select the correct instructions. It also adds a legalizer
test for G_FLOG2.

https://reviews.llvm.org/D57357

llvm-svn: 352673