PR/29798, def forward propagated even though it does not dominate use

[Paolo Bonzini]

This PR is a more complex example of a case that is already covered in 
fwprop's simplistic available expression analysis.  In some obscure 
situations we can have a def reaching a use that is _before_ the def.  
In other words the def does not dominate the use.  This can happen when 
a value from the previous iteration is used in a loop.

We need to hoist the test nearer to the beginning of use_killed_between, 
and this makes us produce good code.

Bootstrapped/regtested i686-pc-linux-gnu.  Ok for mainline?

Paolo
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