[AArch64][GlobalISel] Select FCMPSri/FCMPDri when comparing against 0.0

Add support for selecting FCMPSri and FCMPDri when comparing against 0.0, and
factor out opcode selection for G_FCMP into its own function.

Add a test to show that we don't do this with other immediates.

Differential Revision: https://reviews.llvm.org/D62539

llvm-svn: 361888