[GlobalISel][AArch64] Add G_FCEIL to isPreISelGenericFloatingPointOpcode

If you don't do this, then if you hit a G_LOAD in getInstrMapping, you'll end
up with GPRs on the G_FCEIL instead of FPRs. This causes a fallback.

Add it to the switch, and add a test verifying that this happens.

llvm-svn: 349822