[ARM GlobalISel] Support G_FDIV for s32 and s64

TableGen already generates code for selecting a G_FDIV, so we only need
to add a test.

For the legalizer and reg bank select, we do the same thing as for the
other floating point binary operations: either mark as legal if we have
a FP unit or lower to a libcall, and map to the floating point
registers.

llvm-svn: 318915