[globalisel][legalizerinfo] Add support for the Lower action in...

[globalisel][legalizerinfo] Add support for the Lower action in getActionDefinitionsBuilder() and use it in AArch64.

Lower is slightly odd. It often doesn't change the type but the lowerings
do use the new type to decide what code to create. Treat it like a mutation
but provide convenience functions that re-use the existing type.

Re-uses the existing tests:
test/CodeGen/AArch64/GlobalISel/legalize-rem.mir
test/CodeGen/AArch64/GlobalISel//legalize-mul.mir
test/CodeGen/AArch64/GlobalISel//legalize-cmpxchg-with-success.mir

llvm-svn: 329623