[AArch64][GlobalISel] Optimize G_FCMP + G_SELECT pairs when G_SELECT is fp

Instead of emitting all of the test stuff for a compare when it's only used by
a select, instead, just emit the compare + select. The select will use the
value of NZCV correctly, so we don't need to emit all of the test instructions
etc.

For now, only support fp selects which use G_FCMP. Also only support condition
codes which will only require one select to represent.

Also add a test.

Differential Revision: https://reviews.llvm.org/D62695

llvm-svn: 362446