GlobalISel: record correct stack usage for signext parameters.

The CallingConv.td rules allocate 8 bytes for these kinds of arguments
on AAPCS targets, but we were only recording the smaller amount. The
difference is theoretical on AArch64 because we don't actually store
more than the smaller amount, but it's still much better to have these
two components in agreement.

Based on Diana Picus's ARM equivalent patch (where it matters a lot
more).

llvm-svn: 296754