This patch fixes a bug in which the function prologue would save more 
registers to the stack than there was space allocated. This would cause 
data corruption when the epilogue restored the registers if a child 
function had overwritten that memory.

The problem was caused by insn constraints that allow vectors to be 
placed in scalar registers. This isn't usually allowed without an 
explicit vec_duplicate in the pattern, but some post-reload splitters do 
it sometimes (hence the size calculation mismatch).

A full fix would add vec_duplicate variants of all the instructions that 
support this, but that's a huge explosion of patterns, and this fix is 
enough for correctness, for now.

This has already been committed to the OG12 branch.

Andrew