[Sparc] Don't overlap variable-sized allocas with other stack variables.

[Sparc] Don't overlap variable-sized allocas with other stack variables.

On SparcV8, it was previously the case that a variable-sized alloca
might overlap by 4-bytes the last fixed stack variable, effectively
because 92 (the number of bytes reserved for the register spill area) !=
96 (the offset added to SP for where to start a DYNAMIC_STACKALLOC).

It's not as simple as changing 96 to 92, because variables that should
be 8-byte aligned would then be misaligned.

For now, simply increase the allocation size by 8 bytes for each dynamic
allocation -- wastes space, but at least doesn't overlap. As the large
comment says, doing this more efficiently will require larger changes in
llvm.

Also adds some test cases showing that we continue to not support
dynamic stack allocation and over-alignment in the same function.

llvm-svn: 285131