[ARM] GlobalISel: Support array parameters/arguments

[ARM] GlobalISel: Support array parameters/arguments

Clang coerces structs into arrays, so it's a good idea to support them.
Most of the support boils down to getting the splitToValueTypes helper
to actually split types. We then use G_INSERT/G_EXTRACT to deal with the
parts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304132 91177308-0d34-0410-b5e6-96231b3b80d8