The LLVM IR representation of byval arguments has a rather strange property:...

The LLVM IR representation of byval arguments has a rather strange property: if the alignment of an argument to a call is less than the specified byval alignment for that argument, there is no way to specify the alignment of the implied copy.  Therefore, we must ensure that the alignment of the argument is at least the byval alignment.  To do this, we have to mess with the alignment of relevant alloca's in some cases, and insert a copy that conceptually shouldn't be necessary in some cases.

This patch tries relatively hard to avoid creating an extra copy if it can be avoided (see test3 in the included testcase), but it is not possible to avoid in some cases (like test2 in the included testcase).

rdar://9483886

llvm-svn: 132957