[ARM] GlobalISel: Support struct params/returns

Very very similar to the support for arrays. As with arrays, we don't
support returning large structs that wouldn't fit in R0-R3. Most
front-ends would likely use sret arguments for that anyway.

The only significant difference is that when splitting a struct, we need
to make sure we set the correct original alignment on each member,
otherwise it may get split incorrectly between stack and registers.

llvm-svn: 304536