backend: fix mem2reg & regalloc

they were broken, especially for unstructured control flow. most
significant fix is to register allocator for temporaries that are used
before the first definition in the source order, e.g.:

    @1:
        %x = add %y, 1
        b @3
    @2
        %y = ...
        b @1

it's legal for %x to use %y there (assuming @2 dominates @1) but from
the point of view of the register allocator %y is defined and freed
and then used again, which broke things. the fix is to introduce phis
for this situation:

    @1:
        %y.1 = phi @2 %y
        %x = add %y.1, 1
        b @3
    @2
        %y = ...
        b @1

then regalloc phi handling code makes it work

1 files changed, 1 insertions, 0 deletions

diff --git a/ir.c b/ir.c
index 69cb1fa..79f9ebd 100644
--- a/ir.c
+++ b/ir.c
@@ -25,6 +25,7 @@ struct addr addrht[1 << 12];
 static int naddrht;
 struct xcon conht[1 << 12];
 static int nconht;
+int visitmark;
 
 void
 irinit(struct function *fn)