This patch is part bug fix, part better optimization.

Firstly, my initial patch series introduced a bug that caused an 
internal compiler error when the input to a multiply was a constant. 
This was caused by the gimple verification rejecting such things. I'm 
not totally clear how this ever worked, but I've corrected it by 
inserting a temporary SSA_NAME between the constant and the multiply.

I also discovered that widening multiply-and-accumulate operations were 
not recognised if any one of the three inputs were a constant. I've 
corrected this by adjusting the pattern matching. This also required 
inserting new SSA_NAMEs to make it work.

In order to insert the new SSA_NAME, I've simply reused the existing 
type conversion code - the only difference is that the conversion may be 
a no-op, so it just generates a straight forward assignment.

OK?

Andrew