[ARM GlobalISel] Import MOVi32imm into GlobalISel

Make it possible for TableGen to produce code for selecting MOVi32imm.
This allows reasonably recent ARM targets to select a lot more constants
than before.

We achieve this by adding GISelPredicateCode to arm_i32imm. It's
impossible to use the exact same code for both DAGISel and GlobalISel,
since one uses "Subtarget->" and the other "STI." to refer to the
subtarget. Moreover, in GlobalISel we don't have ready access to the
MachineFunction, so we need to add a bit of code for obtaining it from
the instruction that we're selecting. This is also the reason why it
needs to remain a PatLeaf instead of the more specific IntImmLeaf.

llvm-svn: 351056