[GlobalISel][AArch64] Select G_FFLOOR

This teaches the legalizer about G_FFLOOR, and lets us select G_FFLOOR in
AArch64.

It updates the existing floating point tests, and adds a select-floor.mir test.

Differential Revision: https://reviews.llvm.org/D57486

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353722 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed