Don't allow loop preheaders to have critical edges.

If the block which would be a preheader for a loop has other successors,
don't hoist instructions into it. Instead create a dedicated preheader.

cranelift/filetests/licm/critical-edge.clif

@@ -0,0 +1,42 @@
+test licm
+target riscv32
+
+; The loop in this function is entered from a critical edge.
+
+function %critical_edge(i32, i32) -> i32 {
+
+            ebb0(v0: i32, v7: i32):
+[SBzero#38]   brnz v7, ebb1(v0)
+[Iret#19]     return v0
+
+            ebb1(v1: i32):
+              v2 = iconst.i32 1
+              v3 = iconst.i32 2
+              v4 = iadd v2, v3
+[SBzero#18]   brz v1, ebb2(v1)
+              v5 = isub v1, v2
+[UJ#1b]       jump ebb1(v5)
+
+            ebb2(v6: i32):
+[Iret#19]     return v6
+
+}
+; sameln: function %critical_edge
+; nextln: ebb0(v0: i32, v7: i32):
+; nextln:     brnz v7, ebb3(v0)
+; nextln:     return v0
+; nextln: 
+; nextln: ebb3(v8: i32):
+; nextln:     v2 = iconst.i32 1
+; nextln:     v3 = iconst.i32 2
+; nextln:     v4 = iadd v2, v3
+; nextln:     jump ebb1(v8)
+; nextln: 
+; nextln: ebb1(v1: i32):
+; nextln:     brz v1, ebb2(v1)
+; nextln:     v5 = isub v1, v2
+; nextln:     jump ebb1(v5)
+; nextln: 
+; nextln: ebb2(v6: i32):
+; nextln:     return v6
+; nextln: }