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, 2 insertions, 3 deletions

diff --git a/irdump.c b/irdump.c
index 3d95843..21b5f09 100644
--- a/irdump.c
+++ b/irdump.c
@@ -196,10 +196,9 @@ dumpblk(struct function *fn, struct block *blk)
    for (i = 0; i < blk->phi.n; ++i) {
       struct instr *phi = &instrtab[blk->phi.p[i]];
       union ref *refs = phitab.p[phi->l.i];
-      assert(phi->op == Ophi);
       efmt("    %s ", clsname[phi->cls]);
-      if (!phi->reg) efmt("%%%d = phi ", blk->phi.p[i]);
-      else efmt("(%%%d)%s = phi ", phi - instrtab, mctarg->rnames[phi->reg-1]);
+      if (!phi->reg) efmt("%%%d = %s ", blk->phi.p[i], opnames[phi->op]);
+      else efmt("(%%%d)%s = %s ", phi - instrtab, mctarg->rnames[phi->reg-1], opnames[phi->op]);
       for (int i = 0; i < blk->npred; ++i) {
          if (i) efmt(", ");
          efmt("@%d ", blkpred(blk, i)->id);