Out Of SSA Rewrite - New TER implementation

[Mark Shinwell]

Andrew MacLeod wrote:
> This patch re-implements TER.  The fundamental idea remains the same...
> step through the function determining tracking which single use
> expressions are currently available for substitution,  and marking them
> as replaceable if the use is seen while they are still valid.

I've only had a quick look at this, but I think it might still suffer
from PR# 29877 which we looked at recently.  Since I haven't finished
preparing a mainline patch for that yet (the testcase identified in my
patch on Bugzilla needed further alteration for other targets) then
perhaps I should just hang on until this is in?

Mark